Editing Transmission Control Protocol (section)

==TCP ports==
{{see also|List of TCP and UDP port numbers}}
A TCP connection is identified by a four-[[tuple]] of the source address, source [[port (computer networking)|port]], destination address, and destination port.{{efn|Equivalently, a pair of [[network sockets]] for the source and destination, each of which is made up of an address and a port}}{{sfn|RFC 9293|loc=4. Glossary}}{{sfn|RFC 8095|p=6}} Port numbers are used to identify different services, and to allow multiple connections between hosts.{{sfn|RFC 9293|loc=2.2. Key TCP Concepts}} TCP uses [[16-bit]] port numbers, providing 65,536 possible values for each of the source and destination ports.{{sfn|RFC 9293|loc=3.1. Header Format}} The dependency of connection identity on addresses means that TCP connections are bound to a single network path; TCP cannot use other routes that [[multihomed host]]s have available, and connections break if an endpoint's address changes.{{sfn|Paasch|Bonaventure|2014|p=51}}

Port numbers are categorized into three basic categories: well-known, registered, and dynamic or private. The well-known ports are assigned by the [[Internet Assigned Numbers Authority]] (IANA) and are typically used by system-level processes. Well-known applications running as servers and passively listening for connections typically use these ports. Some examples include: [[File Transfer Protocol|FTP]] (20 and 21), [[Secure Shell|SSH]] (22), [[TELNET]] (23), [[SMTP]] (25), [[HTTPS|HTTP over SSL/TLS]] (443), and [[HTTP]] (80).{{efn|As of the latest standard, [[HTTP/3]], [[QUIC]] is used as a transport instead of TCP.}} Registered ports are typically used by end-user applications as [[Ephemeral port|ephemeral]] source ports when contacting servers, but they can also identify named services that have been registered by a third party. Dynamic or private ports can also be used by end-user applications, however, these ports typically do not contain any meaning outside a particular TCP connection.

[[Network Address Translation]] (NAT), typically uses dynamic port numbers, on the public-facing side, to [[disambiguation|disambiguate]] the flow of traffic that is passing between a public network and a private [[subnetwork]], thereby allowing many IP addresses (and their ports) on the subnet to be serviced by a single public-facing address.