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Chapter 3
Internet Protocols
Total Length: 187
Identifier: 22486
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IP Type: 0x06 TCP
Header Checksum: 0xd031
Source IP Address: 10.7.1.30
Dest. IP Address: 10.7.1.10
No Internet Datagram Options
Can you distinguish the 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 informa-
tion for the next layer, IP wouldn't know what to do with the data carried
in the packet. The example above tells IP to hand the segment to TCP.
Figure 3.7 demonstrates how the Network layer sees the protocols at
the Transport layer when it needs to hand a packet to the upper-layer
protocols.
F I G U R E 3 . 7
The Protocol field in an IP header
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 3.2 is a list of some other popular protocols that can be specified in
the Protocol field.
TCP
UDP
Protocol
numbers
IP
Transport
layer
Internet
layer
17
6
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