Port numbers

An IP address alone isn't quite enough. We need port numbers. To return to the telephone analogy, if IP addresses are phone numbers, then port numbers are like phone extensions.

Generally, an IP address gets a packet routed to a specific system, but a port number is used to route the packet to a specific application on that system.

For example, on your system, you may be running multiple web browsers, an email client, and a video-conferencing client. When your computer receives a TCP segment or UDP datagram, your operating system looks at the destination port number in that packet. That port number is used to look up which application should handle it.

Port numbers are stored as unsigned 16-bit integers. This means that they are between 0 and 65,535 inclusive.

Some port numbers for common protocols are as follows:

Port Number		Protocol
20, 21	TCP	File Transfer Protocol (FTP)
22	TCP	Secure Shell (SSH)	Chapter 11, Establishing SSH Connections with libssh
23	TCP	Telnet
25	TCP	Simple Mail Transfer Protocol (SMTP)	Chapter 8, Making Your Program Send Email
53	UDP	Domain Name System (DNS)	Chapter 5, Hostname Resolution and DNS
80	TCP	Hypertext Transfer Protocol (HTTP)	Chapter 6, Building a Simple Web Client Chapter 7, Building a Simple Web Server
110	TCP	Post Office Protocol, Version 3 (POP3)
143	TCP	Internet Message Access Protocol (IMAP)
194	TCP	Internet Relay Chat (IRC)
443	TCP	HTTP over TLS/SSL (HTTPS)	Chapter 9, Loading Secure Web Pages with HTTPS and OpenSSL Chapter 10, Implementing a Secure Web Server
993	TCP	IMAP over TLS/SSL (IMAPS)
995	TCP	POP3 over TLS/SSL (POP3S)

Each of these listed port numbers is assigned by the Internet Assigned Numbers Authority (IANA). They are responsible for the official assignments of port numbers for specific protocols. Unofficial port usage is very common for applications implementing custom protocols. In this case, the application should try to choose a port number that is not in common use to avoid conflict.