Most people have probably heard of port redirection, for the average home user you may need to do this to increase your download speed for a home torrent! However, port forwarding is used for much more than that, it is used to allow someone to use a service that is behind a NAT router or firewall.
What is NAT – NAT stands for “Network Address Translation”, which is what is used to translate a public ip address to a private one. But why should we do this? Well, when the Internet was first created, no one ever thought we could run out of our 4,294,967,296 IP addresses (Not all of them are globally routed!) It turns out that we underestimated that figure, and a few years ago people started to realize that we are over. This includes NAT; NAT allows multiple machines to use a single public ip address. This means that we can have 1000 users who surf the Internet, all under one public ip address. This obviously takes care of the lack of address space, but not without its shortcomings.
So how about port forwarding? – Well, this really has to do with NAT, because we can already have cases where there are several machines behind a public ip address, what happens when someone sends a web request to the public ip address that hosts a website? How does the router know where to send the request? Yes, you guessed it, with port forwarding! The router will have a list of ports that the administrator can configure to forward to an internal machine. This means that once the router receives a packet destined for port 80, for example, it checks its port forwarding table and sends the packet to the appropriate internal IP address.
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The disadvantage of NAT – Although NAT solves our address space problem (without NAT we would have run out of address space some time ago!), It also created several problems of its own. To begin with, NAT takes the point-to-point connection on which the Internet is based, for example, to say that a malicious request has been sent by a public ip, if you want to trace it back to an individual user, you will have many problems especially if there is more than 100 users! NAT also causes problems with some older software and there are cases when applications and games will not work through a NAT router.
Although NAT has caused several problems, they defiantly do not outweigh the pros. Until ipv6 is fully implemented, NAT will continue to save us from the lack of address space facing the Internet. NAT also offers some security benefits; this is because the NAT router will only forward a packet that has a valid destination, if the port is not forwarded, the packet is dropped.
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