This is the socket descriptor. In the 5-tuple association described above, the socket system call just specifies the first element, the protocol . Before the socket descriptor is of any real use, the remaining four elements of the association must be specified. The bind system call assigns a name to an unnamed socket. There are three uses of bind .
Servers register their well-known address with the system.
Both connection-oriented and connectionless servers need to do this before accepting client requests. A client can register a specific address for itself. In these cases the connect system call does not return until the connection is established, or an error is returned to the process. The client does not have to bind a local address before calling connect . This system call is used by a connection-oriented server to indicate that it is willing to received connections.