A connection between two programs on different computers is represented by its ________.

• A. Pair of IP addresses
• B. Pair of port numbers
• C. Pair of sockets
• D. None of the above

Answer: (C)

A connection between two programs on different computers is identified by the pair of sockets at each end. A socket is the endpoint of a communication on a host, defined by the host’s IP address and the process’s port number. Since a connection involves two endpoints—one on each machine—the relevant representation is the pair of sockets, one on the sending side and one on the receiving side. This pair uniquely identifies the session, because it ties together both ends’ IPs and ports (the full 4-tuple used by TCP). If you only looked at IP addresses or port numbers alone, you couldn’t distinguish which services or which direction the data would flow between the two hosts. The pair of sockets captures the exact endpoints involved in the connection.