A connection between two programs on different computers is represented by which of the following?

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

Answer: (C)

A connection between two programs on different computers is defined by the two endpoints that participate in the communication—each endpoint is a socket on its respective host. Each socket uses the host’s IP address and the port number the program is using, so together the pair uniquely identifies the connection. In TCP, this arrangement is described by the four-tuple: source IP, source port, destination IP, destination port. That’s why a pair of sockets is the best representation: IP addresses or port numbers alone don’t capture both ends of the conversation — they need to be combined with the corresponding end on the other machine to form the actual connection.