10 Killer Open Source Networking Tools
Nmap is a lightweight security scanner that's heavy on utility. It can perform tasks as simple as a ping sweep to see which IP addresses are active and responding, as well as carry off complex scripts to scan your systems for known vulnerabilities. Another fun feature of Nmap is the ability to analyze the reply packets it receives from a host to determine which OS the host is running.
Nmap is available for nearly every platform imaginable and is amazingly useful as a network and security analysis tool.
Nmap is most commonly used to see which services or ports are open or available on a host. It supports both TCP and UDP scanning. You can give it a single host to scan or a CIDR (Classless Inter-Domain Routing) block or an entire list of hosts and networks from a file. A dizzying range of options allows you to specify which types of packets to send out and to see which hosts are susceptible to various remote attacks. Additionally, Nmap provides several options to bypass firewalls and other network filters that would otherwise block your scans.