Highasakite.net & Unclebob.net

Ports.

A port is a communication channel number, over which data transfers take place. For a complete list of all possible port numbers & the appropriate data protocols look at the file /etc/services, here’s a wee sample of some of the important ports that we will be using.

ftp-data	20/tcp	File Transfer [Default Data]
ftp-data	20/udp	File Transfer [Default Data]
ftp	21/tcp	File Transfer [Control]
fsp	21/udp	UDP File Transfer
ssh	22/tcp	SSH Remote Login Protocol
ssh	22/udp	SSH Remote Login Protocol
telnet	23/tcp	telnet
telnet	23/udp	Telnet
smtp	25/tcp	Simple Mail Transfer
smtp	25/udp	Simple Mail Transfer
domain	53/tcp	Domain Name Server
domain	53/udp	Domain Name Server
http	80/tcp	World Wide Web HTTP
http	80/udp	World Wide Web HTTP
pop3	110/tcp	Post Office Protocol - Version 3
pop3	110/udp	Post Office Protocol - Version 3
auth	113/tcp	Authentication Service
auth	113/udp	Authentication Service

All ports below port 1024 are privileged ports, that means that they are reserved for particular types of communication & nothing else. Ports 1024 & over are used for all other types of communication. When you use your Windows PC at home to visit this site your browser sends a request out on a port over 1024 to my port 80, the http daemon on my server opens a port over 1024 & connects this back to the very same port as the request was sent from on your PC, & the server sends it’s answer back to this port. Any other requests that you send will take place over the same port numbers. As soon as the http daemon assigns you to a port on my server over port 1024 it reopens port 80 to accept any incoming requests.

The knowledge of what ports are open, i.e. available for incoming communication, & which daemons are running behind them is important for the overall security & configuration. Again the command netstat –an will show you which ports are open, & what connections are active.