Nmap

Nmap 6.0

Free
Uses raw IP packets in novel ways to determine what hosts are available
4.2  (36 votes)
3.3
7.50 (See all)

Nmap ("Network Mapper") is a free and open source (license) utility for network exploration or security auditing.
Nmap uses raw IP packets in novel ways to determine what hosts are available on the network.
It was designed to rapidly scan large networks, but works fine against single hosts.

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