Call LookupService to obtain the standard port number for the service named by the service parameter. Windows provides a number of standard service names such as ftp, http, finger, or time. Each standard service is associated, by convention, with a specific port number. These standard port numbers allow client sockets to indicate the desired service by requesting a specific port number. They allow server sockets to listen on an expected port number for the service offered. Servers can specify additional services and their associated ports in a SERVICES file. For more information on the SERVICES file, see the Microsoft documentation for Windows sockets. 

InitSocket calls LookupService to obtain the port number for the Service specified by a socket component. 

InitSocket 

Service