Sockets, IP and port addressing

A network socket is an internal endpoint for sending or receiving data within a node on a computer network. Concretely, it is a representation of this endpoint in networking software (protocol stack), such as an entry in a table (listing communication protocol, destination, status, etc.), and is a form of system resource.

In practice socket usually refers to a socket in an Internet Protocol (IP) network (where a socket may be called an Internet socket), in particular for the Transmission Control Protocol (TCP), which is a protocol for one-to-one connections. In this context, sockets are assumed to be associated with a specific socket address, namely the IP address and a port number for the local node, and there is a corresponding socket address at the foreign node (other node), which itself has an associated socket, used by the foreign process. Associating a socket with a socket address is called binding.

Note that while a local process can communicate with a foreign process by sending or receiving data to or from a foreign socket address, it does not have access to the foreign socket itself, nor can it use the foreign socket descriptor, as these are both internal to the foreign node. For example, in a connection between 10.20.30.40:4444 and 50.60.70.80:8888 (local IP address:local port, foreign IP address:foreign port), there will also be an associated socket at each end, corresponding to the internal representation of the connection by the protocol stack on that node. These are referred to locally by numerical socket descriptors, say 317 at one side and 922 at the other. A process on node 10.20.30.40 can request to communicate with node 50.60.70.80 on port 8888 (request that the protocol stack create a socket to communicate with that destination), and once it has created a socket and received a socket descriptor (317), it can communicate via this socket by using the descriptor (317). The protocol stack will then forward data to and from node 50.60.70.80 on port 8888. However, a process on node 10.20.30.40 cannot request to communicate based on the foreign socket descriptor, (e.g. "socket 922" or "socket 922 on node 50.60.70.80") as these are internal to the foreign node and are not usable by the protocol stack on node 10.20.30.40.

A protocol stack, today usually provided by the operating system (rather than as a separate library, for instance), is a set of services that allow processes to communicate over a network using the protocols that the stack implements. The application programming interface (API) that programs use to communicate with the protocol stack, using network sockets, is called a socket API. Development of application programs that utilize this API is called socket programming or network programming.

In the standard Internet protocols TCP and UDP, a socket address is the combination of an IP address and a port number, much like one end of a telephone connection is the combination of a phone number and a particular extension. Sockets need not have a source address, for example, for only sending data, but if a program binds a socket to a source address, the socket can be used to receive data sent to that address. Based on this address, Internet sockets deliver incoming data packets to the appropriate application process.