Port Numbers Explained | Cisco CCNA 200-301



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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.

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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

Resources:

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49 Comments

Conor Base

This is a really informative and fruitful video. I nearly took notes for every sentence in the video.

nikhil barapatre

What if I am hosting more than one Web Application on the same computer(server). Then how would the HTTP request differentiate between the two as both the Web Application would be on port 80 ? Could anybody help me on this ?

ron the don

How do create the pool of ips.. Also how did you calculate the hosts.. Thanks

ron the don

I have come to realise if you can't understand subnet masks and subnetting you can't understand networking outside of your home …. I have been learning about ip addresses for over 1 week now.. πŸ˜­πŸ˜­πŸ˜­πŸ˜­πŸ˜­πŸ˜­πŸ™πŸ˜”…. Wish I was in the room just to ask couple small questions that need comfirming

Ibrahim sallieu Kamara

OMG!!

Some tutorials are just too short but they explained everything you're looking for
Thank you very much

keerthana A

Super… I just tried netstat command got information in details .. Thanks you so much..

Apples Apit

What is the difference between port 7 and port 9, which one should be used for WOL?

Oh wow what a great teacher! I wish I had lecturers like this thank you subbed

ThatBoyLuffy

Crystal clear on the basics ! good explanation with the figures

Mario Yu

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shubham sharma

Great tutorial……just one question, what if multiple host are requesting connection for port 80 ?? how multiple host will connect to that server?

X Deserved It Lol

that’s all cool but how to i know what port to use when stressing a router

Mofo Jackson

Just saw your excellent video explaining TCP UDP and was going to ask if there was one on ports when this auto played haha. Awesome and exactly what I was wanting to know next

Hoang Van Truong

how web server know the src port number of computer when it responds to the computer's request?

Aravind Swami Potluri

What actually generates that random Port, is it the application(like chrome) or something else?

manar Aljadani

You made it much easier to understand. Thank you so much

Andrew DeLave

This was the best explanation I've come across by far. Thank you!

So i attacked port 80 on my friends ip does that afftect his internet or does it affect only his browser

Thanks bro, your way of explaining these complex topics is the best. I learnt a lot from you. Thanks a lot. You earned a new subscriber

SGT Weenie Arms

great explanation, my professor doesn't actually teach anything and tells us to just read the material on netacad. Ive been lost since the second week. your videos have helped me get some clarity from what i have been reading.

It's Janki in English

This is an excellent video!!! OMG it helped me A LOT!!! to understand Ports numbers!!

Jacob Mundane

I struggle to try and remember which port numbers are TCP, UDP…or why they are. This video helps, but I am lost on port numbers.

LiftOrGTFO

Quick question. If a computer can connect to a server, but the server cannot communicate with the computer…why do you think that is? Is it that the port number on the computer's firewall isn't open to allow connection?

Denise Wray

Thank you, your video took the confusion out of understanding what Ports are and how they work.

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