Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 269

•

Protocol —This field is 8 bits long and indicates the upper-layer protocol. Table 6-1 shows
some key protocol numbers.

IP Protocol Numbers Table 6-1

Protocol Number Protocol

1 ICMP

2 IGMP

6 TCP

9 Any IGP, used by Cisco for IGRP

17 UDP

88 EIGRP

89 OSPF

103 PIM

•

Header Checksum —This field is 16 bits long. The checksum is calculated for the IP
header only, without the data. It is recomputed and verified at each point that the IP header
is processed.

•

Source Address —This field is 32 bits long and is the IP address of the sender.

•

Destination Address —This field is 32 bits long and is the IP address of the receiver.

•

IP Options —This field is variable in length. Some options are as follows: security, Loose
Source Routing, Strict Source Routing, Record Route, and Timestamp.

•

Padding —This field is variable and ensures that the IP header ends on a 32-bit boundary.

IP Fragmentation
One of the key characteristics of IP is fragmentation and reassembly. Although the maximum
length of an IP packet can be over 65,000 bytes, many lower-layer protocols do not support such
large maximum transmission units (MTU). For example, the MTU for Ethernet is approximately
1500 bytes. When the IP layer receives a packet to send, it first queries the outgoing interface
to get its MTU. If the size of the packet is greater than the MTU of the interface, the packet is
fragmented.

When a packet is fragmented, it is not reassembled until it reaches the destination IP layer. The
destination IP layer performs the reassembly. Also, any router in the path can fragment a packet,
and any router in the path can fragment a fragmented packet again. Each fragmented packet
receives its own IP header and is routed independently from other packets.