{"id":78845,"date":"2026-09-25T14:04:06","date_gmt":"2026-09-25T14:04:06","guid":{"rendered":"http:\/\/bangla.sitestree.com\/?p=78845"},"modified":"2026-09-25T14:04:07","modified_gmt":"2026-09-25T14:04:07","slug":"the-protocol-header-field-in-ip-packet","status":"publish","type":"post","link":"http:\/\/bangla.sitestree.com\/?p=78845","title":{"rendered":"The Protocol Header Field in IP Packet"},"content":{"rendered":"\n

IPv4 Protocol field<\/strong> tells the receiver what kind of protocol message is inside the IP datagram\u2019s payload.<\/p>\n\n\n\n

So, no\u2014ICMP, IGMP, OSPF, TCP, and UDP are not flags inside the payload<\/strong>. The IPv4 header contains a small numeric field called Protocol<\/strong>, and that number identifies how to interpret the payload.<\/p>\n\n\n\n

For example:<\/p>\n\n\n\n

IPv4 Datagram\n+------------------------+\n| IPv4 Header            |\n| Protocol = 1           |  \u2190 says \"payload is ICMP\"\n+------------------------+\n| ICMP message           |  \u2190 actual payload\n+------------------------+<\/code><\/pre>\n\n\n\n
If the Protocol field is 6<\/code>:<\/p>\n\n\n\n
IPv4 Header\nProtocol = 6\n      \u2193\nPayload = TCP segment<\/code><\/pre>\n\n\n\n
If it is 17<\/code>:<\/p>\n\n\n\n
Protocol = 17\n      \u2193\nPayload = UDP datagram<\/code><\/pre>\n\n\n\n
And similarly:<\/p>\n\n\n\n
IPv4 Protocol value<\/th>Payload contains<\/th><\/tr><\/thead>
1<\/td>ICMP message<\/td><\/tr>
2<\/td>IGMP message<\/td><\/tr>
6<\/td>TCP segment<\/td><\/tr>
17<\/td>UDP datagram<\/td><\/tr>
89<\/td>OSPF packet<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
So the picture means:<\/p>\n\n\n\n
\n
An IP packet can carry different kinds of higher-level or IP-related protocol data inside its payload.<\/strong><\/p>\n<\/blockquote>\n\n\n\n
What does ICMP do there?<\/h3>\n\n\n\n
If:<\/p>\n\n\n\n
Protocol = 1<\/code><\/pre>\n\n\n\n
then the IP payload is an ICMP message<\/strong>.<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
IP Header\nProtocol = 1\n+\nICMP Echo Request<\/code><\/pre>\n\n\n\n
That is what ping<\/code> uses.<\/p>\n\n\n\n
Or:<\/p>\n\n\n\n
IP Header\nProtocol = 1\n+\nICMP Destination Unreachable<\/code><\/pre>\n\n\n\n
ICMP is used for control, error reporting, diagnostics, etc.<\/p>\n\n\n\n
What about OSPF?<\/h3>\n\n\n\n
If:<\/p>\n\n\n\n
Protocol = 89<\/code><\/pre>\n\n\n\n
the IP payload contains an OSPF packet<\/strong>.<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
IP Header\nProtocol = 89\n+\nOSPF Hello packet<\/code><\/pre>\n\n\n\n
Routers use OSPF packets to exchange routing information.<\/p>\n\n\n\n
OSPF does not<\/strong> normally sit inside TCP or UDP.<\/p>\n\n\n\n
It goes directly inside IP:<\/p>\n\n\n\n
IP\n\u2514\u2500\u2500 OSPF<\/code><\/pre>\n\n\n\n
not:<\/p>\n\n\n\n
IP\n\u2514\u2500\u2500 UDP\n    \u2514\u2500\u2500 OSPF<\/code><\/pre>\n\n\n\n
ICMP is similar<\/h3>\n\n\n\n
ICMP also goes directly inside IP:<\/p>\n\n\n\n
IP\n\u2514\u2500\u2500 ICMP<\/code><\/pre>\n\n\n\n
TCP and UDP are:<\/p>\n\n\n\n
IP\n\u2514\u2500\u2500 TCP\n    \u2514\u2500\u2500 Application data<\/code><\/pre>\n\n\n\n
or:<\/p>\n\n\n\n
IP\n\u2514\u2500\u2500 UDP\n    \u2514\u2500\u2500 Application data<\/code><\/pre>\n\n\n\n
That is a very important difference.<\/p>\n\n\n\n
So what is the red dot in the diagram?<\/h3>\n\n\n\n
The red dot represents the idea of multiplexing\/demultiplexing<\/strong>.<\/p>\n\n\n\n
At the sender, IP may receive data from:<\/p>\n\n\n\n
TCP\nUDP\nICMP\nIGMP\nOSPF\n...<\/code><\/pre>\n\n\n\n
and encapsulate whichever one is being sent into the IP payload.<\/p>\n\n\n\n
Then the IPv4 Protocol field<\/strong> identifies which one it is.<\/p>\n\n\n\n
At the receiver:<\/p>\n\n\n\n
IPv4 Protocol field\n        \u2193\n   Is it 1?\n        \u2193\n      ICMP\n\n   Is it 6?\n        \u2193\n      TCP\n\n   Is it 17?\n        \u2193\n      UDP\n\n   Is it 89?\n        \u2193\n      OSPF<\/code><\/pre>\n\n\n\n
So it is essentially saying:<\/p>\n\n\n\n
\n
\u201cWhat kind of data is inside my payload?\u201d<\/strong><\/p>\n<\/blockquote>\n\n\n\n
One small issue with the diagram<\/h3>\n\n\n\n
The diagram visually places ICMP, IGMP, and OSPF around the Network layer, which is reasonable. TCP and UDP belong to the Transport layer.<\/p>\n\n\n\n
So a cleaner conceptual picture would be:<\/p>\n\n\n\n
             TCP       UDP\n              \\         \/\n               \\       \/\n                \\     \/\n                 IP\n              \/  |   \\\n           ICMP IGMP OSPF<\/code><\/pre>\n\n\n\n
But all five can be carried directly in an IPv4 payload<\/strong>, and the IPv4 Protocol field<\/strong> tells the receiver which one is present.<\/p>\n\n\n\n
The key point is:<\/p>\n\n\n\n
\n
Protocol field = identifier<\/strong><\/p>\n\n\n\n
Payload = actual TCP, UDP, ICMP, IGMP, or OSPF message<\/strong><\/p>\n<\/blockquote>\n\n\n\n
It is not<\/strong> that the payload contains all of them at once. Each individual IP packet normally carries one indicated next protocol<\/strong>.<\/p>\n\n\n\n
From AI Tools as is.<\/p>\n","protected":false},"excerpt":{"rendered":"
IPv4 Protocol field tells the receiver what kind of protocol message is inside the IP datagram\u2019s payload. So, no\u2014ICMP, IGMP, OSPF, TCP, and UDP are not flags inside the payload. The IPv4 header contains a small numeric field called Protocol, and that number identifies how to interpret the payload. For example: If the Protocol field … <\/p>\n
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