IP Ports: Internet packets are addressed to an IP number and a port at the IP address. In principle, a single IP Address can have up to 65535 ports active simultaneously. In general, each active port will service only one type of request. There is no absolute requirement that specific services can only operate at specific ports, but when connecting to an unfamiliar server it helps a lot to know in advance what port the desired service will reside at. The alternative is to probe ports randomly or systematically for a service. This practice is not encouraged by system administrators.

Addresses between 0 and 1023 are "Well Known Ports" that will normally have a specific service at a specific port. Usage of these ports is defined by RFC793. Typical "Well known" Ports include: FTP(21), SSH(22), Telnet(23), SMTP(25), DNS(53), DHCP/Bootp(67/68), TFTP(69), HTTP(80), POP2(109), POP3(110), Netbios(137/139), IMAP4(143)

Addresses between 1024 and 41951 are "Registered Ports". They are assigned to known users but can also be used as private or dynamic ports. Some Registered Ports: SOCKS(1080), NFS(2049), Squid(3128), Compuserve (4144)

Addresses above 41951 are Private/Dynamic Ports.

Well Known Ports are listed at: ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers

A good description of port usage is found at: http://www.linuxsecurity.com/resource_files/firewalls/firewall-seen.html

Return To Index Copyright 1994-2002 by Donald Kenney.