{"id":78828,"date":"2026-09-10T02:50:51","date_gmt":"2026-09-10T02:50:51","guid":{"rendered":"http:\/\/bangla.sitestree.com\/?p=78828"},"modified":"2026-09-10T02:50:52","modified_gmt":"2026-09-10T02:50:52","slug":"one-physical-switch-multiple-vlans-how-vlans-work-how-to-configure-them-and-where-ip-subnets-fit-in","status":"publish","type":"post","link":"http:\/\/bangla.sitestree.com\/?p=78828","title":{"rendered":"One Physical Switch, Multiple VLANs: How VLANs Work, How to Configure Them, and Where IP Subnets Fit In"},"content":{"rendered":"\n<h6 class=\"wp-block-heading\">One Physical Switch, Multiple VLANs: How VLANs Work, How to Configure Them, and Where IP Subnets Fit In<\/h6>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>From AI tools as IS.<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A single managed Ethernet switch can behave as though it were several independent switches.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This is the basic idea behind a <strong>VLAN \u2014 Virtual Local Area Network<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Suppose an organization has one 24-port switch. Without VLANs, all 24 ports could belong to the same Layer 2 network. Broadcast traffic generated by one device could potentially reach devices connected to all of the other ports.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">With VLANs, the same physical switch can be divided logically:<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"> <code>                One Physical Switch\n        \u250c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2510\n        \u2502 Ports 1\u20138   \u2192 VLAN 10       \u2502\n        \u2502 Ports 9\u201316  \u2192 VLAN 20       \u2502\n        \u2502 Ports 17\u201323 \u2192 VLAN 30       \u2502\n        \u2502 Port 24     \u2192 Trunk         \u2502\n        \u2514\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2518<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The result is similar to having three separate logical switches inside one physical device.<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10        VLAN 20        VLAN 30\nStaff          Accounting     Guests\n   \u2502               \u2502             \u2502\n   \u2514\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500 One Physical Switch \u2518<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The important question is: <strong>what actually creates the separation?<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It is not the VLAN name. It is not the IP address. It is the <strong>VLAN ID and the switch configuration<\/strong>.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">What Actually Separates One VLAN From Another?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Each VLAN has a numerical identifier.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10\nVLAN 20\nVLAN 30<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Names can optionally be attached:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10 = STAFF\nVLAN 20 = ACCOUNTING\nVLAN 30 = GUESTS<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">But the names are only for administrators.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The actual distinction is:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN ID 10 \u2260 VLAN ID 20 \u2260 VLAN ID 30<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The switch maintains separate Layer 2 forwarding environments for the different VLANs.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">If Port 1 belongs to VLAN 10 and Port 10 belongs to VLAN 20, traffic does not simply flow between those ports merely because they are on the same physical switch.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A broadcast arriving in VLAN 10 remains in VLAN 10.<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC-A\n \u2502\n \u2502 VLAN 10 broadcast\n \u25bc\nSwitch\n \u251c\u2500\u2500 VLAN 10 devices receive it\n \u2514\u2500\u2500 VLAN 20 devices do NOT receive it<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">This is why each VLAN is normally described as a separate <strong>Layer 2 broadcast domain<\/strong>.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">How Does a Switch Know Which VLAN a Device Belongs To?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">For an ordinary user device, the administrator typically assigns the physical switch port to a VLAN.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Port 1 \u2192 VLAN 10\nPort 2 \u2192 VLAN 10\nPort 3 \u2192 VLAN 10\n\nPort 4 \u2192 VLAN 20\nPort 5 \u2192 VLAN 20<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">These ports are commonly called <strong>access ports<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A computer connected to an access port normally does not need to know anything about VLAN tagging.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The computer sends a normal Ethernet frame.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The switch already knows:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Frame entered Port 1\nPort 1 belongs to VLAN 10\nTherefore this frame belongs to VLAN 10<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The VLAN membership is being enforced by the switch.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">How Are VLANs Created?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">On a managed switch, VLANs are normally created through one of several management mechanisms:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>command-line interface;<\/li>\n\n\n\n<li>web interface;<\/li>\n\n\n\n<li>centralized network controller;<\/li>\n\n\n\n<li>cloud management platform;<\/li>\n\n\n\n<li>network-management API.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">For example, on a Cisco-style command-line interface:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>vlan 10\n name STAFF\n\nvlan 20\n name ACCOUNTING\n\nvlan 30\n name GUESTS<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Then ports can be assigned to the VLANs.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>interface GigabitEthernet0\/1\n switchport mode access\n switchport access vlan 10<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Another port could be placed into VLAN 20:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>interface GigabitEthernet0\/2\n switchport mode access\n switchport access vlan 20<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The exact commands vary by manufacturer, but the concept is essentially the same.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The administrator:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>creates a VLAN ID \u2192 assigns ports \u2192 optionally assigns a name \u2192 optionally configures Layer 3 addressing.<\/strong><\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Does Creating a VLAN Require an IP Address?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">No.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This is one of the most important facts about VLANs:<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>A VLAN can exist without any IP address.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<p class=\"wp-block-paragraph\">VLANs operate primarily at <strong>Layer 2<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Ethernet switches can forward Ethernet frames based on MAC addresses without IPv4 or IPv6 being present.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC-A \u2500 Port 1 \u2500\u2510\n               \u2502 VLAN 10\nPC-B \u2500 Port 2 \u2500\u2518<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">PC-A and PC-B could exchange Layer 2 Ethernet frames even if neither has an IP address.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The VLAN still exists.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The switch still keeps its traffic separate from VLAN 20.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Does the Switch Need an IP Address?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">A basic Layer 2 switch does not need an IP address to switch frames.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It can perform:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>MAC learning\n        \u2193\nMAC table lookup\n        \u2193\nEthernet frame forwarding<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">without an IP address assigned to the switch itself.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">However, a <strong>managed switch normally has a management IP address<\/strong> so an administrator can remotely access it using technologies such as:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SSH\nHTTPS\nSNMP\nNetwork controller<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">That IP address is for managing the switch.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It is not what makes Layer 2 switching possible.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A switch could even be configured through a local console connection without having an IP address at all.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Does a VLAN Require IP Subnetting?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">No.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Creating VLANs and creating IP subnets are two separate operations.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A VLAN belongs to Layer 2:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">An IP subnet belongs to Layer 3:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>192.168.10.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Therefore:<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>Subnetting does not create VLANs, and VLAN creation does not automatically create IP subnets.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<p class=\"wp-block-paragraph\">They are separate concepts.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">But in ordinary modern IP networks, they are normally used together.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">The Normal Design: One VLAN, One IP Subnet<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">A common design is:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10 \u2192 192.168.10.0\/24\nVLAN 20 \u2192 192.168.20.0\/24\nVLAN 30 \u2192 192.168.30.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">This means:<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"> <code>         Layer 2                 Layer 3\n\nVLAN 10 \u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2192 192.168.10.0\/24\n\nVLAN 20 \u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2192 192.168.20.0\/24\n\nVLAN 30 \u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2192 192.168.30.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">This is normally the <strong>best and cleanest design<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It gives each Layer 2 broadcast domain its own Layer 3 network.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">VLAN and Subnet Are Not the Same Thing<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">They are often paired so closely that it is tempting to think they are identical.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">They are not.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>VLAN<\/th><th>IP Subnet<\/th><\/tr><\/thead><tbody><tr><td>Layer 2 concept<\/td><td>Layer 3 concept<\/td><\/tr><tr><td>Divides Ethernet networks<\/td><td>Divides IP address space<\/td><\/tr><tr><td>Uses VLAN IDs<\/td><td>Uses network prefixes<\/td><\/tr><tr><td>Example: VLAN 10<\/td><td>Example: <code>192.168.10.0\/24<\/code><\/td><\/tr><tr><td>Determines broadcast domain<\/td><td>Determines which IP addresses are on-link<\/td><\/tr><tr><td>Enforced mainly by switches<\/td><td>Used by hosts and routers<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">A useful way to remember it is:<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>VLAN tells Ethernet which Layer 2 network a device belongs to.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>Subnet tells IP which Layer 3 network an address belongs to.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Can a VLAN Work Without IP?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">Absolutely.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 50<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">may contain equipment communicating through some Layer 2 protocol without using IPv4 or IPv6.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Ethernet itself does not require IP.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A VLAN can also exist simply because an administrator has created it, even if no devices are currently connected.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">So this is perfectly possible:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10\nPorts 1\u20134\nNo IP addresses<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The Layer 2 separation still exists.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">What Changes Once IP Is Introduced?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">Suppose VLAN 10 contains:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC-A\nIP: 192.168.10.10\/24\n\nPC-B\nIP: 192.168.10.20\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Both belong to:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10\nSubnet 192.168.10.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Communication is straightforward.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">PC-A sees that <code>192.168.10.20<\/code> belongs to its own subnet.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It uses ARP to discover PC-B&#8217;s MAC address.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Conceptually:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC-A:\n\nWho has 192.168.10.20?<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The ARP request is an Ethernet broadcast.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Because PC-B is in the same VLAN, it receives the request.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">PC-B responds with its MAC address.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">PC-A can then send:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Ethernet:\nDestination MAC = PC-B\n\nIP:\nDestination IP = 192.168.10.20<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The Layer 2 switch forwards the frame based primarily on the destination MAC address.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">No router is needed.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">What If Two VLANs Use Different IP Networks?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">This is the normal design.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10\n192.168.10.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">and:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 20\n192.168.20.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Suppose:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC-A = 192.168.10.10<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">needs to communicate with:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC-B = 192.168.20.20<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">PC-A realizes:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>192.168.20.20 is NOT in my subnet<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Therefore, it sends the packet to its <strong>default gateway<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">That gateway might be:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>192.168.10.1<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">located on a router or Layer 3 switch.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The path becomes:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC-A\nVLAN 10\n192.168.10.10\n       \u2502\n       \u25bc\nLayer 3 Switch \/ Router\n       \u2502\n       \u25bc\nPC-B\nVLAN 20\n192.168.20.20<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">This is called <strong>inter-VLAN routing<\/strong>.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">What If Two Separate VLANs Use the Same IP Subnet?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">This is where problems begin.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Suppose:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10\nPC-A = 192.168.1.10\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">and:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 20\nPC-B = 192.168.1.20\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Both devices believe they belong to:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>192.168.1.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">PC-A wants to reach:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>192.168.1.20<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">It checks its subnet mask and concludes:<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\">\u201cThat device is on my local network. I don&#8217;t need my router.\u201d<\/p>\n<\/blockquote>\n\n\n\n<p class=\"wp-block-paragraph\">So PC-A sends an ARP broadcast:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Who has 192.168.1.20?<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">But that broadcast exists in:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">PC-B is in:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 20<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">VLAN boundaries prevent the broadcast from crossing.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">PC-B therefore never receives the ARP request.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">PC-A cannot learn PC-B&#8217;s MAC address.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Communication normally fails.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Why Doesn&#8217;t the Router Fix It?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">Because PC-A does not initially send the packet to the router.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">PC-A believes:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>192.168.1.20 = local address<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">So it tries direct Layer 2 delivery.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">That is exactly what the subnet mask tells a host to do.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The host effectively thinks:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Same subnet\n    \u2193\nFind destination MAC\n    \u2193\nSend directly<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">But VLAN separation prevents the necessary Layer 2 discovery.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This creates a mismatch:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>IP says:       SAME NETWORK\n\nVLAN says:     DIFFERENT LAYER 2 NETWORK<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">That is why spreading the same ordinary IP subnet across separate, non-bridged VLANs is normally a bad design.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Could Special Technologies Make It Work?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">Yes.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">There are specialized techniques involving:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>proxy ARP;<\/li>\n\n\n\n<li>Layer 2 bridging;<\/li>\n\n\n\n<li>overlays;<\/li>\n\n\n\n<li>stretched VLANs;<\/li>\n\n\n\n<li>certain data-centre architectures;<\/li>\n\n\n\n<li>specialized gateways.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">But those are exceptions.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">They do not change the fundamental design recommendation:<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>Separate VLANs should normally use separate IP subnets.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">What If One VLAN Contains Multiple IP Subnets?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">Interestingly, the reverse is technically possible.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Suppose one VLAN contains devices from:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>192.168.10.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">and:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>192.168.20.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Both groups exist inside the <strong>same Layer 2 broadcast domain<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This can be made to work.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example, a router or Layer 3 switch could have gateway addresses for both subnets.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">But it is usually undesirable.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Why?<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Because the Layer 2 and Layer 3 boundaries no longer match.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">You might have:<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"> <code>          ONE VLAN\n              \u2502\n      \u250c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2534\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2510\n      \u2502               \u2502\n192.168.10.0\/24   192.168.20.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Both groups share the same Ethernet broadcasts, even though they are logically separate IP networks.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This can make:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>troubleshooting harder;<\/li>\n\n\n\n<li>DHCP design more complicated;<\/li>\n\n\n\n<li>security policies less intuitive;<\/li>\n\n\n\n<li>network documentation confusing;<\/li>\n\n\n\n<li>broadcast domains unnecessarily large.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">So although it can work:<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>Multiple subnets inside one VLAN are normally avoided unless there is a specific operational reason.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">The Cleanest Design<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">The most predictable architecture is:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10\n      \u2502\n      \u2514\u2500\u2500 192.168.10.0\/24\n\nVLAN 20\n      \u2502\n      \u2514\u2500\u2500 192.168.20.0\/24\n\nVLAN 30\n      \u2502\n      \u2514\u2500\u2500 192.168.30.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">This creates matching boundaries:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Layer 2 boundary\n       =\nLayer 3 boundary<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Operationally, that is much easier to understand.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Where Does a Layer 3 Switch Fit In?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">A Layer 2 switch can create VLANs and switch frames within them.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">But it cannot normally route traffic between different IP subnets.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A <strong>Layer 3 switch<\/strong> can do both.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"> <code>       Layer 3 Switch\n       \/       |       \\\n      \/        |        \\\n VLAN 10    VLAN 20    VLAN 30\n    \u2502           \u2502          \u2502\n192.168.10   192.168.20   192.168.30\n .0\/24        .0\/24        .0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The Layer 3 switch can create a logical Layer 3 interface for each VLAN.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">These are commonly called <strong>SVIs \u2014 Switched Virtual Interfaces<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10 gateway = 192.168.10.1\nVLAN 20 gateway = 192.168.20.1\nVLAN 30 gateway = 192.168.30.1<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Then the switch can perform routing between the VLANs.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">One Switch Can Therefore Perform Two Different Jobs<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">A Layer 3 switch might handle traffic in two different ways.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">When two devices are in the same VLAN:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC-A \u2192 Layer 2 switching \u2192 PC-B<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The switch primarily uses MAC addresses.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">When devices are in different VLANs\/subnets:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC-A\n   \u2193\nLayer 3 routing\n   \u2193\nPC-C<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">the switch examines IP addresses and performs routing.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">So the same physical device can perform:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Layer 2 switching\n+\nLayer 3 routing<\/code><\/pre>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">What About VLANs Across Multiple Switches?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">VLANs are not limited to one physical switch.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Suppose:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Switch A                  Switch B\n---------                 ---------\nVLAN 10                   VLAN 10\nVLAN 20                   VLAN 20\nVLAN 30                   VLAN 30<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">One physical connection between the switches can carry traffic from several VLANs.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This is normally done using a <strong>trunk<\/strong>.<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Switch A\n   \u2502\n   \u2502  VLAN 10\n   \u2502  VLAN 20\n   \u2502  VLAN 30\n   \u2502\n   \u2502  Trunk\n   \u2502\nSwitch B<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Ethernet frames travelling across a trunk commonly contain an <strong>IEEE 802.1Q VLAN tag<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Conceptually:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Ethernet Frame\n\nDestination MAC\nSource MAC\nVLAN ID = 20\nPayload<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The VLAN ID tells the receiving switch which logical network the frame belongs to.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Multiple VLANs can therefore share the same physical cable while remaining logically separated.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Access Port vs. Trunk Port<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">These two terms are fundamental.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">An <strong>access port<\/strong> normally carries traffic for one VLAN and connects to an ordinary end device:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>PC\n \u2502\n \u2502 Access port\n \u2502 VLAN 10\n \u25bc\nSwitch<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">A <strong>trunk port<\/strong> normally carries multiple VLANs:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Switch\n   \u2502\n   \u2502 VLAN 10\n   \u2502 VLAN 20\n   \u2502 VLAN 30\n   \u2502\n   \u25bc\nSwitch<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Trunks are commonly used between:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>switches;<\/li>\n\n\n\n<li>switches and routers;<\/li>\n\n\n\n<li>switches and servers;<\/li>\n\n\n\n<li>switches and virtualization hosts;<\/li>\n\n\n\n<li>switches and wireless infrastructure.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Why Use Multiple VLANs on One Switch?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">Consider an office with employees, servers, security cameras, and guests.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Without VLANs:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Employees\nServers\nGuests\nCameras\n      \u2502\n      \u25bc\nOne large Layer 2 network<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">With VLANs:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10 \u2192 Employees\nVLAN 20 \u2192 Servers\nVLAN 30 \u2192 Guests\nVLAN 40 \u2192 Cameras<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The organization gains logical separation without purchasing four completely separate sets of switches.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It can also apply different:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>security policies;<\/li>\n\n\n\n<li>firewall rules;<\/li>\n\n\n\n<li>routing policies;<\/li>\n\n\n\n<li>DHCP configurations;<\/li>\n\n\n\n<li>access controls.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Employees \u2192 Servers       ALLOW\n\nGuests \u2192 Internet         ALLOW\n\nGuests \u2192 Servers          BLOCK\n\nCameras \u2192 Camera Server   ALLOW\n\nCameras \u2192 Employee PCs    BLOCK<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">VLANs provide the Layer 2 separation, while routers, Layer 3 switches, and firewalls provide controlled communication between the resulting networks.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Could We Simply Avoid IP Completely?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">Yes, technically.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">You could have:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10\nVLAN 20\nVLAN 30<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">with no IPv4 or IPv6 configured.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The Ethernet segmentation would still exist.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">But most modern corporate applications depend on IP.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Without IP, devices generally cannot participate normally in:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>web applications;<\/li>\n\n\n\n<li>Internet access;<\/li>\n\n\n\n<li>DNS;<\/li>\n\n\n\n<li>email;<\/li>\n\n\n\n<li>cloud applications;<\/li>\n\n\n\n<li>most client\/server applications.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">So although VLANs <strong>do not technically require IP<\/strong>, practical modern enterprise VLANs are normally paired with IPv4 and\/or IPv6 subnets.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Which Design Is Best?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">For an ordinary modern IP network, the preferred design is generally:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>ONE VLAN\n   \u2193\nONE IP SUBNET\n   \u2193\nONE LAYER 3 GATEWAY<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10\n192.168.10.0\/24\nGateway 192.168.10.1<\/code><\/pre>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 20\n192.168.20.0\/24\nGateway 192.168.20.1<\/code><\/pre>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 30\n192.168.30.0\/24\nGateway 192.168.30.1<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">This aligns:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Layer 2 segmentation<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">with:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Layer 3 addressing<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">and makes routing predictable.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Which Alternatives Create Problems?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">The four basic possibilities can be summarized this way:<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Design<\/th><th>Possible?<\/th><th>Usually recommended?<\/th><\/tr><\/thead><tbody><tr><td>VLAN with no IP subnet<\/td><td>Yes<\/td><td>Yes for special Layer 2 purposes<\/td><\/tr><tr><td>One VLAN + one subnet<\/td><td>Yes<\/td><td><strong>Best normal design<\/strong><\/td><\/tr><tr><td>One VLAN + multiple subnets<\/td><td>Yes<\/td><td>Usually no<\/td><\/tr><tr><td>Multiple isolated VLANs + same subnet<\/td><td>Normally problematic<\/td><td><strong>Avoid<\/strong><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">The problematic case is particularly important:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10 \u2500\u2510\n         \u251c\u2500\u2500 192.168.1.0\/24   \u2190 BAD NORMAL DESIGN\nVLAN 20 \u2500\u2518<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">because IP believes the devices are local to one another while Ethernet has separated their broadcasts.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The cleaner design is:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10 \u2192 192.168.10.0\/24\n\nVLAN 20 \u2192 192.168.20.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Now Layer 2 and Layer 3 agree.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Does Using the Same IP Subnet Across VLANs Cause \u201cChaos\u201d?<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">\u201cChaos\u201d may be an informal word, but it captures the practical problem reasonably well.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Several confusing symptoms can result:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>ARP failures.<\/strong> Devices believe another address is local but cannot reach its Layer 2 broadcast domain.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>IPv6 Neighbor Discovery failures.<\/strong> IPv6 has a similar local-neighbour discovery requirement.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Intermittent connectivity.<\/strong> Depending on gateways and special configurations, some communication may appear to work while other communication fails.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>DHCP confusion.<\/strong> Separate VLANs typically require appropriate DHCP scopes and relay configuration.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Duplicate addresses.<\/strong> Administrators may accidentally reuse addresses because the VLANs appear isolated.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Difficult troubleshooting.<\/strong> IP addressing suggests one topology while the actual Layer 2 topology says something different.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Good network architecture tries to make different layers reinforce rather than contradict each other.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">The Three Boundaries to Keep Separate<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">A useful mental model is:<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Physical boundary<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Which cable or switch port is being used?<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Physical switch port<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">Layer 2 boundary<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Which VLAN does the frame belong to?<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN ID<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">Layer 3 boundary<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Which IP subnet does the address belong to?<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>IP network prefix<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">These are three different questions.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For example:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Physical Port 5\n       \u2193\nVLAN 20\n       \u2193\n192.168.20.0\/24<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">They are related by configuration, but they are not inherently the same thing.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">A Complete Example<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">Suppose one physical switch serves three departments.<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"> <code>             ONE PHYSICAL SWITCH\n\n       \u250c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2510\n       \u2502                          \u2502\n       \u2502 VLAN 10                  \u2502\n       \u2502 Staff                    \u2502\n       \u2502 192.168.10.0\/24          \u2502\n       \u2502                          \u2502\n       \u2502 VLAN 20                  \u2502\n       \u2502 Accounting               \u2502\n       \u2502 192.168.20.0\/24          \u2502\n       \u2502                          \u2502\n       \u2502 VLAN 30                  \u2502\n       \u2502 Guests                   \u2502\n       \u2502 192.168.30.0\/24          \u2502\n       \u2502                          \u2502\n       \u2514\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u252c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2518\n                    \u2502\n               Layer 3\n               Routing\n                    \u2502\n                Firewall\n                    \u2502\n                 Internet<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Ports might be configured as:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Ports 1\u20138   \u2192 VLAN 10\nPorts 9\u201316  \u2192 VLAN 20\nPorts 17\u201323 \u2192 VLAN 30\nPort 24     \u2192 uplink\/trunk<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">IP addressing could be:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 10\n192.168.10.0\/24\nGateway: 192.168.10.1<\/code><\/pre>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 20\n192.168.20.0\/24\nGateway: 192.168.20.1<\/code><\/pre>\n\n\n\n<pre class=\"wp-block-code\"><code>VLAN 30\n192.168.30.0\/24\nGateway: 192.168.30.1<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Within each VLAN:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Layer 2 switching and MAC addresses<\/strong> handle local Ethernet delivery.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Between VLANs:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Layer 3 routing and IP addresses<\/strong> handle communication.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Toward external networks:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>routers\/firewalls<\/strong> provide connectivity and security.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h1 class=\"wp-block-heading\">Final Thought<\/h1>\n\n\n\n<p class=\"wp-block-paragraph\">A single managed Ethernet switch can support many VLANs because a VLAN is a <strong>logical Layer 2 boundary rather than a separate physical switch<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The separation is created through:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>VLAN IDs + switch-port assignments + VLAN tags where required.<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It does not depend on the VLAN name.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It does not depend on IP addressing.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">And it does not require subnetting simply to exist.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">However, once ordinary IP communication is introduced, the cleanest architecture is normally:<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>One VLAN \u2192 one IP subnet \u2192 one Layer 3 gateway.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<p class=\"wp-block-paragraph\">A VLAN can exist without IP.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A subnet can exist without a VLAN.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">One VLAN can technically contain multiple subnets.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Special technologies can even stretch addressing in unusual ways.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">But for ordinary networks, matching one subnet to one VLAN avoids unnecessary complexity.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The key is to keep the layers conceptually separate:<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>VLAN controls Layer 2 membership and broadcasts.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>Subnet controls Layer 3 IP addressing.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>Switches forward Ethernet frames within VLANs.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>Routers or Layer 3 switches route IP packets between subnets.<\/strong><\/p>\n<\/blockquote>\n\n\n\n<p class=\"wp-block-paragraph\">When those boundaries are designed to match each other, the network becomes much easier to understand, operate, secure, and troubleshoot.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>One Physical Switch, Multiple VLANs: How VLANs Work, How to Configure Them, and Where IP Subnets Fit In From AI tools as IS. A single managed Ethernet switch can behave as though it were several independent switches. This is the basic idea behind a VLAN \u2014 Virtual Local Area Network. Suppose an organization has one &hellip; <\/p>\n<p><a class=\"more-link btn\" href=\"http:\/\/bangla.sitestree.com\/?p=78828\">Continue reading<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_post_was_ever_published":false},"categories":[1978],"tags":[],"class_list":["post-78828","post","type-post","status-publish","format-standard","hentry","category-computer-networks","item-wrap"],"jetpack_sharing_enabled":true,"jetpack-related-posts":[{"id":78826,"url":"http:\/\/bangla.sitestree.com\/?p=78826","url_meta":{"origin":78828,"position":0},"title":"Network Devices, VLANs, Subnets, Layer 2 and Layer 3 Switches, and Routers","author":"Sayed","date":"September 10, 2026","format":false,"excerpt":"Network Devices, VLANs, Subnets, Layer 2 and Layer 3 Switches, and Routers From AI tools as IS. Modern computer networks are built from several different kinds of devices and several different kinds of logical boundaries. This can become confusing because terms such as switch, router, VLAN, subnet, LAN, Layer 2,\u2026","rel":"","context":"In &quot;Computer Networks&quot;","block_context":{"text":"Computer Networks","link":"http:\/\/bangla.sitestree.com\/?cat=1978"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":24315,"url":"http:\/\/bangla.sitestree.com\/?p=24315","url_meta":{"origin":78828,"position":1},"title":"11 :: CCNA R&amp;S Exam Course :: Collision Domains, Broadcast Domains, &amp; VLANs #Root","author":"Author-Check- Article-or-Video","date":"April 6, 2021","format":false,"excerpt":"From: http:\/\/sitestree.com\/?p=3142 Categories:RootTags: Post Data:2015-10-27 05:55:44 Shop Online: https:\/\/www.ShopForSoul.com\/ (Big Data, Cloud, Security, Machine Learning): Courses: http:\/\/Training.SitesTree.com In Bengali: http:\/\/Bangla.SaLearningSchool.com http:\/\/SitesTree.com 8112223 Canada Inc.\/JustEtc: http:\/\/JustEtc.net (Software\/Web\/Mobile\/Big-Data\/Machine Learning) Shop Online: https:\/\/www.ShopForSoul.com\/ Medium: https:\/\/medium.com\/@SayedAhmedCanada","rel":"","context":"In &quot;FromSitesTree.com&quot;","block_context":{"text":"FromSitesTree.com","link":"http:\/\/bangla.sitestree.com\/?cat=1917"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":78447,"url":"http:\/\/bangla.sitestree.com\/?p=78447","url_meta":{"origin":78828,"position":2},"title":"Which Router to Use in GNS3","author":"Sayed","date":"September 1, 2025","format":false,"excerpt":"Here\u2019s a copyright-free and plagiarism-free version of Which Router to Use in GNS3: Which Router to Use in GNS3 When you start building labs in GNS3, one of the first choices you\u2019ll face is which router type to use. Each option has different features, requirements, and performance considerations. 1. Cisco\u2026","rel":"","context":"In &quot;Computer Networks&quot;","block_context":{"text":"Computer Networks","link":"http:\/\/bangla.sitestree.com\/?cat=1978"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":78821,"url":"http:\/\/bangla.sitestree.com\/?p=78821","url_meta":{"origin":78828,"position":3},"title":"PAN, LAN, CAN, MAN, and WAN: Understanding Network Types by Geographic Scope","author":"Sayed","date":"September 9, 2026","format":false,"excerpt":"PAN, LAN, CAN, MAN, and WAN: Understanding Network Types by Geographic Scope REf: AI Tools as IS Computer networks are often classified according to the geographic area they cover. The most common categories are: PAN \u2014 Personal Area Network LAN \u2014 Local Area Network CAN \u2014 Campus Area Network MAN\u2026","rel":"","context":"In &quot;Computer Networks&quot;","block_context":{"text":"Computer Networks","link":"http:\/\/bangla.sitestree.com\/?cat=1978"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":16393,"url":"http:\/\/bangla.sitestree.com\/?p=16393","url_meta":{"origin":78828,"position":4},"title":"If you want to learn security, or if you already are a security professional\/engineer, can you answer the following questions on Security? Do you know the answers to the following questions on Security?","author":"Sayed","date":"November 16, 2019","format":false,"excerpt":"Defensive Security If you want to learn security, or if you already are a security professional\/engineer, can you answer the following questions on Security? Do you know the answers to the following questions on Security? True or false, all email servers in the Internet are configured properly? What can you\u2026","rel":"","context":"In &quot;\u09ac\u09cd\u09b2\u0997 \u0964 Blog&quot;","block_context":{"text":"\u09ac\u09cd\u09b2\u0997 \u0964 Blog","link":"http:\/\/bangla.sitestree.com\/?cat=182"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":26254,"url":"http:\/\/bangla.sitestree.com\/?p=26254","url_meta":{"origin":78828,"position":5},"title":"If you want to learn security, or if you already are a security professional\/engineer, can you answer the following questions on Security? Do you know the answers to the following questions on Security? #Root","author":"Author-Check- Article-or-Video","date":"April 21, 2021","format":false,"excerpt":"Defensive Security If you want to learn security, or if you already are a security professional\/engineer, can you answer the following questions on Security? Do you know the answers to the following questions on Security? True or false, all email servers in the Internet are configured properly? What can you\u2026","rel":"","context":"In &quot;FromSitesTree.com&quot;","block_context":{"text":"FromSitesTree.com","link":"http:\/\/bangla.sitestree.com\/?cat=1917"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"jetpack_featured_media_url":"","_links":{"self":[{"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=\/wp\/v2\/posts\/78828","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=78828"}],"version-history":[{"count":1,"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=\/wp\/v2\/posts\/78828\/revisions"}],"predecessor-version":[{"id":78829,"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=\/wp\/v2\/posts\/78828\/revisions\/78829"}],"wp:attachment":[{"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=78828"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=78828"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/bangla.sitestree.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=78828"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}