CCNA Cisco Certified Network Associate Study Guide - FTP Server

120 Chapter 3 � Internet Protocol
Notice that there are logical, or IP, addresses in this header.
The type field—it’s typically a protocol field, but this analyzer sees it as a
type field—is important. If the header didn’t carry the protocol information
for the next layer, IP wouldn’t know what to do with the data carried in the
packet.
Figure 3.7 shows how the Network layer sees the protocols at the Transport
layer when it needs to hand a packet to the upper-layer protocols.
FIGURE 3.7 The protocol field in an IP header
Transport
layer
Internet
layer
In this example, the protocol field tells IP to send the data to either TCP
port 6 or UDP port 17 (both hex addresses). However, it will only be UDP
or TCP if the data is part of a data stream headed for an upper-layer service
or application. It could just as easily be destined for ICMP (Internet Control
Message Protocol), ARP (Address Resolution Protocol), or some other type
of Network layer protocol.
Table 3.1 is a list of some other popular protocols that can be specified in
the protocol field.
TABLE 3.1 Possible Protocols Found in the Protocol Field of an IP Header
Protocol Protocol Number
ICMP 1
IGRP 9
IPv6 41
GRE 47
TCP UDP
6
IP
17
Protocol
numbers