Editing Network address translation (section)

===Establishing two-way communication===
[[File:Bidirectional NAT-en.svg|thumb|In bidirectional NAT the session can be established both from inside and outside realms.]]
Every TCP and UDP packet contains a source port number and a destination port number. Each of those packets is encapsulated in an IP packet, whose [[IP header]] contains a source IP address and a destination IP address. The IP address/protocol/port number triple defines an association with a [[network socket]].

For publicly accessible services such as web and mail servers the port number is important. For example, port 443 connects through a socket to the [[web server]] software and port 465 to a mail server's [[SMTP]] [[Daemon (computer software)|daemon]].<ref>{{IETF RFC|8314}}</ref> The IP address of a public server is also important, similar in global uniqueness to a postal address or telephone number. Both IP address and port number must be correctly known by all hosts wishing to successfully communicate.

Private IP addresses as described in RFC 1918 are usable only on private networks not directly connected to the internet. Ports are endpoints of communication unique to that host, so a connection through the NAT device is maintained by the combined mapping of port and IP address. A private address on the inside of the NAT is mapped to an external public address. Port address translation (PAT) resolves conflicts that arise when multiple hosts happen to use the same source port number to establish different external connections at the same time.