Ccna routing and Switching Complete Study Guide

105

TCP Session: Source Port

Let’s take a minute to check out analyzer output showing a TCP session I captured with my 

analyzer software session now:

TCP - Transport Control Protocol

 Source Port:      5973

 Destination Port: 23

 Sequence Number:  1456389907

 Ack Number:       1242056456

 Offset:           5

 Reserved:         %000000

 Code:             %011000

      

Ack is valid

      

Push Request

 Window:           61320

 Checksum:         0x61a6

 Urgent Pointer:   0

 No TCP Options

 TCP Data Area:

 vL.5.+.5.+.5.+.5  76 4c 19 35 11 2b 19 35 11 2b 19 35 11

  2b 19 35 +. 11 2b 19

Frame Check Sequence: 0x0d00000f

Notice that the source host makes up the source port, which in this case is 5973. The 

destination port is 23, which is used to tell the receiving host the purpose of the intended 

connection (Telnet).

By looking at this session, you can see that the source host makes up the source port by 

using numbers from 1024 to 65535. But why does the source make up a port number? To 

differentiate between sessions with different hosts because how would a server know where 

information is coming from if it didn’t have a different number from a sending host? TCP 

and the upper layers don’t use hardware and logical addresses to understand the sending 

host’s address as the Data Link and Network layer protocols do. Instead, they use port 

numbers.