The previous sections of the Developer's Guide described what information a program needs to communicate over a TCP/IP network. The next step is for the program to create what is called a socket, a communications end-point that can be likened to a telephone. However, creating a socket by itself doesn’t let you exchange information, just like having a telephone in your house doesn’t mean that you can talk to someone by simply taking it off the hook. You need to establish a connection with the other program, just as you need to dial a telephone number, and to do this you need the socket address of the application that you want to connect to. This address consists of three key parts: the protocol family, Internet Protocol (IP) address and the service port number.

We’ve already discussed IP addresses and service ports, but what’s the protocol family? It’s a number which is used to logically designate the group that a given protocol belongs to. Since the socket interface is general enough to be used with several different protocols, the protocol family tells the underlying network software which protocol is being used by the socket. In our case, the Internet Protocol family will always be used when creating sockets. With the protocol family, IP address of the system and the service port number for the program that you want to exchange data with, you’re ready to establish a connection.

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