nmap

Nmap is free port scanner designed to scan what ports and services are opened in a host or a network. The use of nmap is controversial. On one hand, network administrators need it as a diagnostics tool to ensure no unnecessary ports or services are inadvertently opened by mistake. They can also check if malicious ports are being established as a way of detecting any hosts being compromised.

Equally, crackers can use nmap to scan ports, services and what OS version in a hosts or a network. That's why nmap is regarded as a threat by some network adninistrators. The reality is that if there is a host hooked up with the Internet, people can find out what ports and services it is running, even if the port numbers are changed to peculiar ones.