Ccna routing and Switching Complete Study Guide

Port Numbers

TCP and UDP must use port numbers to communicate with the upper layers because 

these are what keep track of different conversations crossing the network simultaneously. 

Originating-source port numbers are dynamically assigned by the source host and will 

equal some number starting at 1024. Port number 1023 and below are defined in RFC 

3232 (or just see 

www.iana.org

), which discusses what we call well-known port numbers.

Virtual circuits that don’t use an application with a well-known port number are 

assigned port numbers randomly from a specific range instead. These port numbers identify 

the source and destination application or process in the TCP segment.

The Requests for Comments (RFCs) form a series of notes about the Inter-

net (originally the ARPAnet) started in 1969. These notes discuss many 

aspects of computer communication, focusing on networking protocols, 

procedures, programs, and concepts, but they also include meeting notes, 

opinions, and sometimes even humor. You can find the RFCs by visiting 

www.iana.org

.

Figure 3.14 illustrates how both TCP and UDP use port numbers. I’ll cover the different 

port numbers that can be used next:

 

■

Numbers below 1024 are considered well-known port numbers and are defined in RFC 

3232.

 

■

Numbers 1024 and above are used by the upper layers to set up sessions with other 

hosts and by TCP and UDP to use as source and destination addresses in the segment.

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TCP/IP and the DoD Model