Port is simply a channel of communication which is numbered between 1 and 65536.It is a kind of subaddress. The IP address is the primary address and the port is the subaddress. Each port is associated with a specific process or service. For example:Port 80 is assosciated for HTTP messages.
Making thing simpler, just think of the IP address as the street address of a building and then the port as the apartment number. I need the street address to get to the right building, but I need the apartment address to find who I am looking for. This is the same with ports. The IP address gets us to right machine, but the port takes us to correct service, say HTTP on port 80.
There are 65,536 ports. The first 1,024 are generally referred to as the common ports.
Some of the ports are:
|80||Hypertext Transfer Protocol (HTTP)|
|443||HTTP Secure (HTTPS) HTTP over TLS/SSL|
|20||File Transfer Protocol (FTP) Data Transfer|
|21||File Transfer Protocol (FTP) Command Control|
|22||Secure Shell (SSH)|
|23||Telnet - Remote login service, unencrypted text messages|
|25||Simple Mail Transfer Protocol (SMTP) E-mail Routing|
|53||Domain Name System (DNS) service|
|119||Network News Transfer Protocol (NNTP)|
|123||Network Time Protocol (NTP)|
|143||Internet Message Access Protocol (IMAP) Management of Digital Mail|
|161||Simple Network Management Protocol (SNMP)|
|194||Internet Relay Chat (IRC)|