Internet Port Numbers
Ports provide a means by which computers communicate with one another using standard protocols. Each protocol is assigned to a fixed port number so that the computers know which port to check in order to communicate using the specified protocol. When you start up a program that will communicate with remote computers using a specific protocol, the port assigned to that protocol is the one that it will use to attempt to communicate. The port number being used needs to be the same on each computer attempting to communicate in order that the message packets sent by the one computer are correctly received and recognised by the other.
Port numbers fall into three ranges. Ports 0 through 1023 are called "Well Known Ports" and are assigned to their specified purpose by the Internet Assigned Numbers Authority (IANA). Ports 124 through 49151 are available for general use and may be registered with IANA if desired. Ports 49152 through 65535 are used where a dynamic or private port is required.
You can protect your computer against some forms of attack over the internet by restricting the ports that your computer allows other computers to use to communicate with it. For maximum security, you should ensure that access to all ports, except those that you need to use, are blocked.
Some of the "Well Known" ports include:
- 20 ftp data
- 21 ftp control
- 22 ssh
- 23 telnet
- 25 smtp (simple mail transfer protocol)
- 53 domain name server
- 70 gopher
- 79 finger
- 80 http (hyper text transfer protocol)
- 107 remote telnet or SNA gateway
- 110 pop (post office protocol)
- 113 identification/authorisation
A complete list of well known ports can be found on the IANA web site.
The following are some other ports you may find referenced:
- 135 RPC
- 139 Netbios
- 143 IMAP
- 443 HTTPS
- 445 MSFTDS
- 5000 UPnP
- 8044 xcom (not registered with IANA)
- 8080 http alternate port
