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Chapter 2
Protocol Attributes
entire header has six layers, and each layer consists of 32 bits. Look at each
section of the header and get an explanation for each.
IP Header - Internet Protocol Datagram
Version:
4
Header Length:
5
Precedence:
0
Type of Service:
%000
Unused: %00
Total Length: 60
Identifier: 0
Fragmentation Flags: %000
Fragment Offset: 0
Time To Live: 2
IP Type: 0x58 IGRP
Header Checksum: 0x10dc
Source IP Address: 205.124.250.7
Dest. IP Address: 224.0.0.10
No Internet Datagram Options
At this point, we will define each of the key fields that appear above. As
you can see, the packet IP header starts out with the Version field. Right
now, the standard is IPv4. The version parameter uses four bits of the 32 bits
available.
The next field is the IP Header Length, or IHL. This field also uses
another four bits and it specifies the datagram header length in 32-bit words.
The Type of Service (TOS) follows the IHL. This field uses eight bits
and indicates datagram priority and how other OSI layers are to handle the
datagram once they receive it.
Following the TOS is the Total Length parameter. This field indicates
how long the packet is, including header and payload or data. The length is
in units of bytes. The field itself uses 16 bits, which brings the total for the
first layer to 32 bits. Because 32 bits have been used, move on to the next
layer.
The second layer begins with the Identifier or identification field.
The Identifier is a 16-bit field that contains an integer value that identifies
the packet. It is like a sequencing number that is used when reassembling
datagram fragments.
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