In the TCP and UDP protocols used in computer networking, a port is a special number present in the header of a data packet. Ports are typically used to map data to a particular process running on a computer. As an example, a server used for sending and receiving email may provide both an SMTP and a POP3 service; these will be handled by different server processes, and the port number will be used to determine which data is associated with which process. This may be considered loosely analogous to simulating the effect of a single server with multiple physical connections. Note that not all transport layers use network ports; for example, although UDP and TCP use ports, ICMP does not.