5.1 IP Datagram

In explaining the TCP/IP protocol family, it is common to draw everything in a table that has a line of 4 bytes, i.e., bits 0 to 31. We too will often refer to this description.

An IP datagram consists of a header and transmitted data. A header usually has 20 bytes. However, a header can also contain optional entries that can make it longer.

The structure of an IP datagram is shown in the following figure:

Figure 5.7: IP datagram

Before we start to describe particular header entries, we will capture an IP datagram using MS Network Monitor. This way we will immediately be able to see whether what we are describing is really being transmitted through the network. Now we can begin to look at the meaning of individual entries in the IP datagram header.

+ FRAME: Base frame properties
+ ETHERNET: ETYPE = 0x0800 : Protocol = IP: DOD Internet Protocol
IP: ID = 0x5814; Proto = ICMP; Len: 60
IP: Version = 4 (0x4)
IP: Header Length = 20 (0x14)
IP: Service Type = 0 (0x0)
IP: Precedence...