Remember when we sent a packet to PC911? Well, we actually sent two port numbers, a source port (the port number of the browser window that you are running) and a destination port. The destination port was port 80, because port 80 is reserved for the Hyper Text Transport Protocol (HTTP, web page servers) and we were accessing a web page.
e-mail servers listen for Simple Mail Transport Protocol (SMTP) requests on port 25. Domain Name Servers (DNS, they resolve host names to IP addresses) listen for DNS requests on port 53. Unlike the random port numbers assigned to client side applications like web browsers, all services have unique ports reserved for them -and those ports do not change. For example: an http server (an Internet site like PC911) will always listen for requests on port 80.
The server that holds your e-mail will always listen to port 25 to receive e-mail from other e-mail servers. If you download your e-mail to your computer from your e-mail server, then your request went to port 110 (Post Office Protocol, POP). The application that you use to read e-mail was assigned a random port number above port number 1023 (just like your web browser) and your e-mail server used that port number to reply to you.
To review: All of the Ethernet packets that you send from your computer to the Internet have two port numbers, one source and one destination. The source port number is the port number of the application that generated the Ethernet packet, and it can be any number greater than 1023. The destination port number is the port number of the service that you are requesting, and it will be a port number below 1024. All port numbers below 1024 are reserved for specific services.
Every application on your computer is assigned a unique port number so that when data returns from the Internet, the Transport Layer on your PC knows which application generated the request for data and the information gets passed to the correct application. You wouldn’t want Da Lan Tech loading into your PC911 window, would ya’?