The Internet Assigned Numbers Authority (IANA) assigns port numbers. IANA is a standards body that is responsible for assigning various addressing standards.

There are different types of port numbers, as shown in Figure 1:

Figure 2 displays some common well-known and registered ports within TCP. Figure 3 displays common well-known and registered ports within UDP.

Using both TCP and UDP

Some applications may use both TCP and UDP (Figure 4). For example, the low overhead of UDP enables DNS to serve many client requests very quickly. Sometimes, however, sending the requested information may require the reliability of TCP. In this case, the well-known port number, 53, is used by both TCP and UDP with this service.

A current list of port numbers and the associated applications can be found on the IANA organizational website.