IP Header - Fields, Length, TTL, Protocol, and Checksum
The IP header carries the information routers and hosts need to deliver a packet across networks. In IPv4, the header includes version, length, fragmentation fields, TTL, protocol, checksum, and addresses.
Key Takeaways
IPv4 header length is variable because options can extend it.
TTL prevents packets from circulating forever.
The Protocol field tells whether the payload is TCP, UDP, ICMP, or another protocol.
Fragmentation fields explain split packets and reassembly behavior.
What the IP header does
The IP header describes how a packet should move between networks. It does not provide application meaning by itself; instead, it points to the next protocol and carries addressing and routing metadata.
Important IPv4 fields
The most important fields for analysis are Version, IHL, Total Length, Identification, Flags, Fragment Offset, TTL, Protocol, Header Checksum, Source Address, and Destination Address.
How to read the Protocol field
The IPv4 Protocol field tells the parser what comes next. Common values include TCP, UDP, and ICMP. If this value is misread, the rest of the packet will be decoded incorrectly.
Practical Reference
| Item | Value | Analysis Note |
|---|---|---|
| IHL | Header length | Multiply by 4 to get bytes. |
| TTL | Hop limit | Decrements at each router. |
| Protocol | Next payload type | TCP and UDP are common. |
| Checksum | IPv4 header integrity | Covers the header only. |
FAQ
Is IP header the same as IPv4 header?
Often people mean IPv4 header when they say IP header. IPv6 has a different fixed-size base header.
Why does IPv4 have a header checksum?
The checksum helps detect corruption in the IPv4 header. It must be updated when routers decrement TTL.