Ccna routing and Switching Complete Study Guide

TCP Session: Destination Port

You’ll sometimes look at an analyzer and see that only the source port is above 1024 and 

the destination port is a well-known port, as shown in the following trace:

TCP - Transport Control Protocol

 Source Port:      1144

 Destination Port: 80 

World Wide Web HTTP

 Sequence Number:  9356570

 Ack Number:       0

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106

 

Chapter 3 

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  Introduction to TCP/IP 

 Offset:           7

 Reserved:         %000000

 Code:             %000010

      

Synch Sequence

 Window:           8192

 Checksum:         0x57E7

 Urgent Pointer:   0

 TCP Options:

  Option Type: 2 

Maximum Segment Size

    Length:    4

    MSS:       536

  Option Type: 1 

No Operation

  Option Type: 1 

No Operation

  Option Type: 4

    Length:    2

    Opt Value:

  No More HTTP Data

Frame Check Sequence: 0x43697363

And sure enough, the source port is over 1024, but the destination port is 80, indicat-

ing an HTTP service. The server, or receiving host, will change the destination port if it 

needs to.

In the preceding trace, a “SYN” packet is sent to the destination device. This Synch (as 

shown in the output) sequence is what’s used to inform the remote destination device that it 

wants to create a session.