Ccna routing and Switching Complete Study Guide

Header checksum    Cyclic redundancy check (CRC) on header only.

Source IP address    32-bit IP address of sending station.

Destination IP address    32-bit IP address of the station this packet is destined for.

Options    Used for network testing, debugging, security, and more.

Data    After the IP option field, will be the upper-layer data.

Here’s a snapshot of an IP packet caught on a network analyzer. Notice that all the 

header information discussed previously appears here:

IP Header - Internet Protocol Datagram

 Version:             4

 Header Length:       5

 Precedence:          0

 Type of Service:     %000

 Unused:              %00

 Total Length:        187

 Identifier:          22486

 Fragmentation Flags: %010 

Do Not Fragment

 Fragment Offset:     0

 Time To Live:        60

 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

The Type field is typically a Protocol field, but this analyzer sees it as an IP Type field. 

This 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. The preceding example 

clearly tells IP to hand the segment to TCP.

Figure 3.16 demonstrates how the Network layer sees the protocols at the Transport 

layer when it needs to hand a packet up to the upper-layer protocols.

f I g u r e   3 .16     The Protocol field in an IP header

TCP

UDP

IP

Transport

layer

Port numbers

Network

layer

6

17

---

TCP/IP and the DoD Model 

111

In this example, the Protocol field tells IP to send the data to either TCP port 6 or UDP 

port 17. But it will be UDP or TCP only 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 Internet Control 

Message Protocol (ICMP), Address Resolution Protocol (ARP), or some other type of 

Network layer protocol.

Table 3.3 is a list of some other popular protocols that can be specified in the Protocol 

field.

Ta b l e   3 . 3     Possible protocols found in the Protocol field of an IP header