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In the last video, we looked at the layer 4 protocols TCP and UDP. Layer 4 is also responsible for choosing port numbers.
So why do we need port numbers? The example I give here is the postal service. To get post, you install a letterbox (at least here in the UK). This letterbox allows the postal service to access your home.
Network applications work in the same way. If you want to communicate over a network, you must allow some means of access to your device.
Let’s say we want to access a web server. We type in the web address (also known as a URL) of the site we want to visit.
The first thing the computer does it convert that URL to an IP address.
This is done by using DNS, but we will cover than in another video. For now, just know it coverts web addresses to IP addresses.
The computer then sends the request to the webserver. The server will then look at the destination port to find out which application to send this data to. It can do this because there is a standard that port numbers follow.
When we made our web request, our computer knew we were trying to access a HTTP site. So it added the destination port number 80 to the TCP header.
The computer will also choose a randomly generated source port to receive reply’s on.
Several port numbers are called well-known ports. These are common protocols that have been assigned port numbers.
Examples of well-known port numbers are HTTP(80), HTTPS(443), SMTP (25).
0 – 1023: Well-known port numbers.
1024 – 49151: Registered ports that companies have registered IANA
49152-65535: Dynamic ports
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