Port numbers are assigned to different protocols and services so that a user will know how and what to access at the other end and to identify specific applications. They are also used to track the session associated with that protocol. As explained in Chapter 1, the TCP and UDP headers include a 16-bit source and destination port, so port numbers can be represented in the range of 0 to 65535 (the maximum decimal number that can be represented on 16 bits).
Port numbers fall into two categories:
- Well-known port numbers
- Temporary port numbers
Well-known port numbers are non-ephemeral ports, meaning they are permanently assigned to a protocol or a service. They can be used as a destination port by any network-enabled machine in order to reach specific protocols. A well-known port number helps to uniquely identify a Web server, for example. Even though the Web server might use a random port number, it usually uses port number 80 as a convention so other devices can know how to reach it. If the service used a non-standard port number, a client device could reach it only if it was specifically informed about the port number used. Well-known port numbers are usually those below 1024.
Temporary port numbers are ephemeral ports and are usually used as source ports in a two-way communication process. They are determined in real-time by the client workstation and are usually numbers above 1024. When the session is closed, the generated source port number disappears and a new session destined to the same service will usually use a different source port. The process of generating ephemeral port numbers is done automatically by the TCP/IP stack on the devices and it uses sequential or random port numbers. The allocation process of the unique source ports is accomplished at the Application Layer.
| Note: The TCP header contains port numbers but they are really being used by the Session Layer of the OSI reference model (i.e., bundled in the Application Layer in the TCP/IP protocol suite).
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Even though the same port number can be used in both TCP and UDP, the services they identify can be completely different. For example, TCP port 80 identifies a different service/protocol than UDP port 80 does.
Do not confuse port numbers with protocol numbers. Port numbers uniquely identify network protocols, while protocol numbers identify Layer 3 protocols, for example:
- TCP is protocol number 6
- UDP is protocol number 17
- ICMP is protocol number 1
- IGMP is protocol number 2