TCP/IP and the DoD Model
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The following fields make up the IP header:
Version IP version number.
HLEN Header length in 32-bit words.
Priority or ToS
Type of Service tells how the datagram should be
handled. The first three bits are the priority bits.
Total length Length of the packet including header and data.
Identification Unique IP-packet value.
Flags Specifies whether fragmentation should occur.
Frag offset Provides fragmentation and reassembly if the packet is too
large to put in a frame. It also allows different maximum transmission
units (MTUs) on the Internet.
TTL The time to live is set into a packet when it is originally generated.
If it doesn't get to where it wants to go before the TTL expires, boom--
it's gone. This stops IP packets from continuously circling the network
looking for a home.
Protocol
Port of upper-layer protocol (TCP is port 6 or UDP is
port 17 [hex]).
Header checksum Cyclic redundancy check (CRC) on header only.
Source IP address 32-bit IP address of sending station.
Destination IP address 32-bit IP address of the station this packet is
destined for.
IP option Used for network testing, debugging, security, and more.
Data Upper-layer data.
Here's a snapshot of an IP packet caught on a network analyzer. Notice
that all the information discussed above appears here:
IP Header - Internet Protocol Datagram
Version: 4
Header Length: 5
Precedence: 0
Type of Service: %000
Unused: %00
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