1.1.5. Port

Ports are address components used in network protocols to assign the datagrams to the right services (applications). They are expressed as numbers between 0 and 65535. Every number is assigned to a certain service, e.g. port 80 is assigned to the application "Webserver". The next picture illustrates these connections.

Port

On your computer a file exists with all port numbers and some descriptions at:

• Windows NT: C:/WINDOWS/system32/drivers/etc/services
• Unix and Mac: /etc/services

Example: Sending an E-Mail to a friend

To understand better the function of the port we make an example of sending an e-mail to a friend:

When clicking on the send-button the e-mail is sent by TCP to a Mail-Server using the port 25 and is saved on the Mail-Server until the friend downloads his e-mails. If he does so, his computer sends a request to the Mail-Server with TCP using the port 143. The content of the e-mail is then transferred with TCP to his e-mail program.

Sending an E-Mail

If you still don't understand the function of the port number have a look at the following popup solutions. (Click here for more information)