All the operating systems now honor the tradition of permitting only the super-user open the ports numbered 0 to 1023.  These well-known ports (also called standard ports) are assigned to services by the IANA (Internet Assigned Numbers Authority, www.iana.org).   On Unix, the text file named /etc/ services (on Windows 2000 the file named %windir%\ system32\ drivers\ etc\ services) lists these service names and the ports they use.  Here are a few lines extracted from this file:

echo             7/tcp     Echo
ftp-data         20/udp    File Transfer [Default Data]
ftp              21/tcp    File Transfer [Control]
ssh              22/tcp    SSH Remote Login Protocol
telnet           23/tcp    Telnet
domain           53/udp    Domain Name Server
www-http         80/tcp    World Wide Web HTTP