nmap - how to scan hosts of networks for open ports

nmap is a wonderful tool specially for debugging, there are lots of times when you need to know if a port is open in a server, or maybe blocked by a firewall, or just to test your iptables rules. Here we will learn how to use it at the command line, and using its GUI front end, nmapFE and Knmap.

Introduction

Well, so what does nmap does?

From the man page:

Nmap (“Network Mapper”) is an open source tool for network exploration and security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts.