Port Numbers

Notes:

Services are ‘advertised’ by having individual servers listen for communications on a particular endpoint called a port. It is normal to have server listen on the same port on all hosts that provide the service. This is sometimes called the rendezvous port. For example, to contact the Telnet service, you would communicate, using TCP, with port number 23 on that system.
Port numbers between 1 and 511 are reserved for standard TCP/IP applications such as FTP and Telnet. This is so that these services can always be contacted at the same port number, regardless of the IP address of the system.
Most operating systems additionally reserve port numbers between 512 and 1023 for use by privileged programs. This means that locally developed network programs must use port numbers above 1024.
Management of the standard TCP/IP port numbers is carried out by the central TCP/IP authority. Local port-number management is the job of the network administrator.
Many services are available on both TCP and UDP. In these cases, the process normally uses the same port number with both protocols.
Client processes require endpoints for communication, just as servers do. However, it is not important for a client always to use the same port number. As long as a port number is used consistently within a conversation with the server, any one will do. To allow this, TCP and UDP can dynamically select an unused port (usually with a high number) to give to a client if required. These are known as ephemeral port numbers.
Firewalls can be configured to filter TCP connections based on port numbers, for example outgoing TCP segments using port 25 (Email) from any internal host can be passed but incoming segments on that port only allowed a the mail relay host.