Nmap
Nmap or network mapper is an open source tool used for gathering information about a network and systems on it. Systems on a network are assigned an IP address. Much like an apartment building has a street address in a city. Applications and services that run on these systems might need to communicate with other applications or services on the same or on different networks. They do this by programmatically designating a certain part of the network packet address to a particular service. This designation is known as a port. In our apartment building example, ports are the mailboxes for all the people who live in the building. Apache, the most popular web server globally, will be monitoring or “listening” to packets addressed to port 80 and/or 443 on the machine that is hosting this application. The packets with port 80 and 443 in the destination port section of the packet are intended for Apache. All other packets with other ports in the destination address of the packet could be for an...