An IP address can locate a host, but many different programs can run on the host. For example, there are Web servers (websites), databases and FTP servers at the same time. If a request only specifies an IP address, it is impossible to find the correct program, so add the port number. Each program has a specific port number. For example, if the port number of the HTTP server is 8080, then the HTTP request is (IP address): 8080, so as to find the correct service program.