(Port range forwarding) Explain this simple router feature, what it does, and how i would use it

I'm trying to figure out "all the other" things about the routers, I just have never had a need to poke at these things.

In this image there is port forwarding, and range port forwarding, explain to me:

Why would I need to specify that 'all' communication on a certain port would need to go to a specific address, wouldn't the incoming packages know where they were going?

What application (except games) needs to have certain ports forwarded, I thought it was only firewalls that dealt with ports as far as open or close them. Why do I need to forward packages and why and what application needs this (real life example)

This might seem like a self explanatory feature at first glance, but i would like to know a little more about it (and not Google it). A few pointers would be highly appreciated.

I'm trying to understand the concept, not the specific feature in this specific router.
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somewhereinafricaAsked:
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maeltarCommented:
This is to allow INBOUND traffic to a specific host on your network, rather than opening the router and forwarding all ports to one host, you can map each port to a specific host, say for example you have a web server using port 80 on a PC in your office with an IP of 192.168.1.1

Your start-end port would be 80
To IP address would be 192.168.1.1
Enabled "checked"

This would allow external users (public internet) to access your webserver if they know what the public IP address of your router is.

Likewise, say you had an FTP server an a different PC with an IP of 192.168.1.200

Your start-end port would be 21
To IP address would be 192.168.1.200
Enabled "checked"

So your NAT (Network Address Translation) table will translate incoming requests (From public internet) and for want of a better way of putting it, direct them to the correct destination inside your private network..
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SuperdaveCommented:
"wouldn't the incoming packages know where they were going?"

No, and that is the whole point of it.  The incoming packages don't know where they are going because their destination address is the address of the router.

Any program that listens on a port needs this, or the incoming packages won't reach the computer.  This would include most things called "servers", such as ssh server, web server, ftp server; also bit torrent clients which are peer-to-peer basically.

If your computer initiates the connection, the packets coming back from the other end need to know where to go back to, but the router keeps track of this when your computer makes the connection so you don't have to do anything special to set it up.
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somewhereinafricaAuthor Commented:
aaahhhhhhh, if the incoming package was not initiated by someone on the LAN, how would the router know where on the network to send it.

THAT simple, I could just for the life of me not think outside the box on that one. Thank you oodles
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