IP address/TCP port combination

A combination of attributes that uniquely identifies Simple Mail Transfer Protocol (SMTP), Network News Transfer Protocol (NNTP), Internet Message Access Protocol (IMAP), and Post Office Protocol (POP) virtual servers in Exchange Server 2003. Virtual servers can share an Internet Protocol (IP) address, provided their TCP ports are different; if they share a TCP port, their IP addresses must be different. The combination must be unique on all virtual servers. This is also true for Hypertext Transfer Protocol (HTTP) virtual servers, except that they have a third unique identifying characteristic: a host name. [Exchange 2003]