Useful TCP well-known port numbers

If you are working with TCP/IP services regularly, it is useful to remember a few common TCP port numbers.

These are some of the most frequently used services:

21   FTP
23   Telnet
25   SMTP
53   DNS
80   HTTP
110  POP3
119  NNTP
143  IMAP
443  HTTPS

For example, a web server normally listens on port 80:

http://server/

SMTP mail servers usually listen on port 25:

telnet mail.server.com 25

NNTP servers use port 119, which is commonly used by news reader applications.

Knowing these ports makes troubleshooting much easier when testing connectivity or configuring firewalls. I think it's a good idea to memorize at least the most common ones.