why do we need ports ?
Answers
Answer:
Ports are needed so that traffic coming from different applications on different sources can simultaneously reach the same host and when there is more than one source of a particular type of traffic for one host, then a new port is created for that traffic that will only link to that host.
What are ports and why are they important?
I like to think of networking in terms of real life. So if the computer is my home, IP Address is the street number, and MAC the house number. But that's not all you need to enter my house, is it?
You need doors too, or maybe a window, to enter the house.
Thats what ports are.
Now generally a house has doors on one of the vertical walls. So you circle around the house, find such a door and try to enter. These are the well-defined ports, whose port numbers are fixed, and any basic port scan will scan these ones.
But what if I have an underground door. Or maybe I have a door on the horizontal wall(roof). These are the other port numbers.
So your machine will know the well defined port numbers, but if you want to connect to a server, say foo, but foo has moved its http server from port 80 to port 34251, then you will be unable to find it, as by default, your browser will try to connect to port 80 and find it closed.
I hope this helps.