Scanning the network for open ports and services

Scanning a network is an important part of pentesting. We scan a network to find loopholes and then exploit them to gain access to computer systems that are part of that network.There are number of attacks which works on a network but we are not going to discuss them yet,we will be focusing on finding open ports and services in a network or in a system attached to that network.

What is Port?
Whenever two computers on a same network want to communicate with each other,they do so with the help of ports.A port is an extension of an address, similar to adding an apartment or room number to a street address. A letter with a street address will arrive at the correct apartment building, but without the apartment number, it will not be delivered to the correct recipient. Ports work in much the same way. A packet can be delivered to the correct IP address, but without the associated port, there is no way to determine which application should act on the packet.There are 65,535 ports available but we need only few of them.Follow are some basic ports along with their port number:-
1 ) HTTP(Hyper Text Transfer Protocol) - 80
2 ) FTP(File Transfer Protocol) - 21
3 ) SMTP(Simple Mail Transfer Protocol)- 25
4 ) TELNET - 23
5 ) SSH - 22
6 ) POP3(Post Office Protocol-Version 3)- 110
7 ) 140-159 - Unassigned
8 ) 160-223 - Reserved
You can read more about these ports on internet.