A+: Ports

For two computers to communicate they must both use the same protocol. In order for an application to send or receive data it must use a particular protocol designed for that application, and open up a port on the network adapter to make a connection to another computer. For example, let us say you wished to visit www.google.com. You would open up a browser and type http://www.google.com. The protocol being used is HTTP, short for Hypertext Transfer Protocol. That is the protocol that makes the connection to the web server: google.com. The HTTP protocol would select an unused port on your computer (known as an outbound port) to send and receive data to and from google.com. On the other end, google.com’s web server will have a specific port open at all times ready to accept sessions. In most cases the web server’s port is 80, which corresponds to the HTTP protocol. This is known as an inbound port.

If the port is blocked, the service it depends on will fail.

Port Protocol
21 FTP
22 SSH
23 Telnet
25 SMTP
53 Kerberos
80 HTTP
110 POP3
443 HTTPS
1863 Kinect Sensor
3074 XBOX 360