Using nmap to scan Ports

If you need to know, which ports are open on a system you can scan it with the open source tool nmap.

A simple
nmap [ip or domain]
will result in a list of open/filtered ports.

nmap -sT [ip or domain]
will scan the TCP-Ports.

nmap -sU [ip or domain]
will scan the UDP-Ports.

nmap -p 20-400 [ip or domain]
will check all ports between 20 and 400.