How does work to expose your PC from behind a router?

Posted on 2003-11-09
Last Modified: 2010-04-17
How does work to expose your PC from behind a router?  As I understand it, the router issues internal addresses like 10.0.?.? or 192.168.?.? which are not routable over the internet.

So how do they do it?

Question by:JimHubbard
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Expert Comment

ID: 9711563
Hi Jim,
there is two ways of doing it: Network Address Translation (NAT) or Port Address Translation (PAT). Both expression are also used for the translation of addresses in traffic FROM private networks to the public. For incoming NAT and PAT we almost always talk about "static", - i.e static NAT and static PAT, - meaning that the routing is always predictable.

In both cases, the router does the job.

In NAT, you tell the router to send all trafic directed to [public address] on to the internal host with [private address]. In this case you will need 1 public address for each private address, so not much profit in that.

With PAT, the router looks at the requested port in incoming traffic, and based on that, reroutes the traffic to a specific host on the private subnet. I.e. incoming traffic aimed at port 80 (the http or 'web' port) can be directed to an internal server.

With PAT you can have 1 server for each port you want to use, - e.g. 1 web-server, 1 mail-server, 1 ftp-server, and so on.

Kind regards,

Author Comment

ID: 9711606

   I understand what you are saying, but the software from does not alter any router settings internally (AFAIK).  So how do they pull it off?


Accepted Solution

svenkarlsen earned 500 total points
ID: 9711669
you cannot control a router without configuring it. A router decides what goes where and why, so if you imagine a router setup not to allow any incoming traffic, then, regardless of how you configure a computer on the private network, it will not get any incoming traffic to 'manipulate'

I'm  sorry Jim, - I read your question without focusing on the "":

Their offer is not to handle what I mentioned under NAT & PAT, - they offer to handle situations where your public IP is constantly changing (because you havent bought a 'static' IP at your ISP.

If you get an account at some ISP with a 'dynamic' IP address, that means that your public IP is unpredictable, - i.e. the ISP is allowed to change it whenever they feel like it (which they will do, bcause they want you to pay for a static IP ;-)

Now, - if you have a domain-name (say: and you have a webserver on that domain, then has to point to your IP address if people should be able to reach you by typing in their browser. But your IP keeps changing if you got a dynamic public IP, so what do you do ?

Well then you go to someone like or similar (there's many providers of this service), and they make sure to keep track of your current public IP and to publish it as the IP of

I know this sounds a bit complicated, but it all boils down to: what do you want to do (and this is a question for you):

1. do you want to have a public web-(or other-) server at your home ?
2. are you just curious ?

If 1.: try talking to your ISP about it, - explain your needs and get his prises. Then check-out 2 or 3 alternatives before you make any decisions.

if 2.: search the net for "IP names howto", "IP dns howto", "IP names tutorial", "IP dns tutorial" and similar. There's a lot of info out there and by searching for tutorials you will find many excellent entry-point papers on most subjects.

Kind regards,

Author Comment

ID: 9711804

I have 3 servers at home off a cable connection, with 4 static IPs and 3 other machines on a DSL account with dynamic IPs.

I guess I misunderstood their software (wish i could submit a screenshot) where it asks if you are on a LAN with a static IP.  I inferred that they meant the static IP was an internal one, because that's the one they show next to the question AND because if you have a static external IP, why would you be using their service?

Thanks for your replies!


Expert Comment

ID: 9711822
The services is providing, is more typically known as "Dynamic DNS" (remember this if you need to search for providers on the net).

if you need to make your internal boxes publicly available, then it's your router you should focus on. Check out your documentation and search for "static", - that's the keyword, because various vendors tend to use other names than NAT and PAT. It is known to be called 'tunnels' , 'links' and whathaveyounot....;-)

Regards (& thx for pts!)


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