IP protocol consists of a series of sub-protocols, such as TCP, UDP, ARP and ICMP. Among them, TCP and UDP protocols are mainly used to transmit data. In both TCP and UDP protocols, the concept of port number exists. The function of port number is mainly to distinguish business categories and conduct multiple sessions at the same time.

For example, a host A needs to provide FTP and WWW services, if there is no port number.

In other words, there is no difference between the two services. In fact, when a host B on the network needs to access the FTP service of A, it needs to specify the destination port number as 21; When you need to visit A's WWW service, you need to set the destination.

The port number is set to 80. At this point, A can distinguish two different requests of B according to the port number accessed by B ... This is the role of port number in distinguishing service categories.