Network - (Internet) Socket (Network identifier)


A socket is session identifier for a network connection to or from which information is transmitted in the network between a server (local machine) and a client (remote machine).

This connection is implemented by a software (not hardware) by creating a network interface.

A socket is therefore an endpoint of an inter-process communication flow across a computer network.

Today, most communication between computers is based on the Internet Protocol; therefore most network sockets are Internet sockets (ie TCP socket).

Socket communications are nowadays bidirectional (full duplex) (With Tcp sure, udp ???)


An socket on a operating system level is characterized by a unique combination of the following:

The output of netstat showing a socket Id with its state. They are all TCP socket


A socket address is the combination of

  • the protocol: TCP or UDP
  • and an endpoint (host+port)


The type of socket is defined by the protocol that implements it. (ie a TCP socket is a socket implemented by TCP)

There are several Internet socket types available:

  • Datagram sockets, also known as connectionless sockets
  • Stream sockets, also known as connection-oriented sockets, which use Transmission Control Protocol (TCP) or Stream Control Transmission Protocol (SCTP).
  • Raw sockets (or Raw IP sockets), typically available in routers and other network equipment. Here the transport layer is bypassed, and the packet headers are made accessible to the application.

There are also non-Internet sockets, implemented over other transport protocols, such as:

  • Systems Network Architecture (SNA).
  • Unix domain sockets (UDS), for internal inter-process communication.


Socket lifecycle.

A state is an information that is given for a protocol that supports a connection. It means that this is often about a TCP socket because UDP is connection less


  • Listening: Computer processes that provide application services are called as servers, and create sockets on start up that are in listening state. These sockets are waiting for initiatives from client programs. See TCP Listening state


The operating system forwards the payload of incoming IP packets to the corresponding application by extracting the socket address information from the IP and transport protocol headers and stripping the headers from the application data.



The output of netstat showing a socket Id with its state


  • Number of open sockets with netstat
netstat | grep port



  • Java: For IPC, you could also use ObjectOutputStream and ObjectInputStream connected to network sockets and exchange Java objects directly between the two programs. Or you could design your own protocol.

1) 2) 3)

original definition of socket given in RFC 147, as it was related to the ARPA network in 1971

Powered by ComboStrap