A transport-layer protocol in the TCP suite needs both the IP address
and the port number, at each end, to make a connection. The
combination of an IP address and a port number is called a socket
The client socket address defines the client process uniquely just as
the server socket address defines the server process uniquely as
shown in Figure.
To use the services of transport layer in the Internet, we need a pair of socket addresses: the client socket address and the server socket address.
These four pieces of information are part of the network-layer packet
header and the transport-layer packet header. The first header
contains the IP addresses; the second header contains the port