{"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":71266,"url":"http:\/\/bangla.sitestree.com\/?p=71266","url_meta":{"origin":78828,"position":0},"title":"Difference: Auto-Regressive and Auto Correlation.","author":"Sayed","date":"September 27, 2021","format":false,"excerpt":"If you can, answer the question below: Write your answer in the comment box. 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