Nmap

Nmap 5.5

Free
Nmap can perform network inventory, list a service and identify firewall types
4.2  (36 votes)
3.3
7.50 (See all)

Nmap is documented extensively online. You can find it all at the Nmap documentation directory.
Nmap can perform network inventory, list a service history, identify firewall types and pocket filers, and detect the uptime of the system and the OS. Its huge database may identify the OS fingerprints of the target system easily and reliably.

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