This information is going to mostly apply to DSL users, but this will also apply to to cable users if you've been issued a router, not just a modem, but a router by your ISP. (From a cable company usually this router will be wireless). This post will only apply for cable users who have two routers on their network.
When most people sign up for DSL service, their ISP will send them a "modem" to connect to the telephone line and allow them to plug their computer into the Internet. Generally these "modems" only have a single outlet on them to allow one computer to be plugged in, and they're usually not wireless capable.
So, when the user wants to add more computers, or to connect wireless laptops or video games, they need to go out and buy additional equipment to expand the capabilities of the hardware provided by the ISP.
Unfortunately, what many people don't realize is that the "modem" provided by the ISP is often a ROUTER too. The ISPs usually don't make it clear what the unit is, and some of them are actually labeled as "modem" when they're really combination units with a router built-in as well.
This means that when the user goes out to the store and buys a "wireless router" to add wireless to the system, or a "router" to add more LAN sockets, they're actually BLOCKING their ports with TWO routers.
All of this applies to cable users too, if they are running two routers in their network. You just have to be a bit more discerning if you are a cable user. Don't worry, there is a pretty easy way to find out if you are running behind two routers if you are unsure. I'll show you later.
While it's possible to set up port forwarding between two routers, there are more practical ways to set up your network that will save you time and frustration.
The CORRECT thing to buy, if you already have a Combination Modem/Router is either an "ethernet switch" to add more LAN sockets, or a "Wireless Access Point" to add wireless to the existing setup. Both of these devices will NOT block anything and will allow your port forwarding to work successfully.
The problem is, hardly anyone knows this, and the people at the store have no clue about it either. So most people will end up buying the WRONG THING and finding later that they can't get port forwarding to work for games or filesharing.
Connecting two routers together WILL allow "normal" Internet web surfing and e-mail to work, but any more "advanced" applications will be solidly BLOCKED.
So- what to do, if you've got a separate "modem" and have gone out and added a router, then found you can't get port forwarding to work..?
The FIRST thing to do is to find out if your "modem" is really a combination modem/ROUTER.
If your "modem" has more than one LAN socket, or if it's wireless, it's most likely a ROUTER.
If you know how to "log in" to the modem and see anything that says "Port Forwarding" or "Virtual Server", it's a ROUTER.
If it says Speedstream, Speedtouch, Westell, or Zyxel strongly suspect that it's a "combination modem/ROUTERS". There ARE other brands that can be set up as a router, and these can sometimes be configured as "just a modem", so the brand isn't conclusive.
The BEST way to tell if your "modem" is actually configured as a router is to log into your SECOND router, the one you've added to your setup, and look at the "Status page". The router's Status page should display the "WAN IP" address, sometimes called "Internet address" that the router is using.
(You might want to write it down.)
The SECOND router will be getting this IP address from the FIRST device. So by looking at the IP address, we can tell if the FIRST device is configured as a router, or if it's just a modem.
If the SECOND router's WAN IP or Internet IP address is any one of these:
172.16.0.0 - 172.31.255.255
Then your "modem" is actually configured as a ROUTER, which means it will be BLOCKING the ports you need.
If the IP address is the same as your "Public IP address" (click the link at the top of this page that says PortForward's HOME to go to the Port Forwarding home page and look right before the list of routers to find your "Public IP address"), then it means the "modem" is actually just a modem and is passing everything through without blocking anything.
So now that you know what your "modem" really is, what now..?
If it's just a modem, there's no problem. Just go ahead and do the port forwarding procedure for the router and things should work OK.
If it's actually a "Combination Modem/Router", the BEST solution is to take the SECOND ROUTER, the one you've bought to add to the system, back to the store and exchange it for the CORRECT device.
As mentioned earlier, to add more LAN sockets, you need an "ethernet switch", to add Wireless capability you need a "Wireless Access Point". Installing either of these devices will be simpler than the alternative procedure listed below.
Unfortunately, you may no longer be able to take the unit back, or would rather not hassle with the store, or are reluctant to go back to your Dad and tell him the wireless router you begged him to buy is the wrong thing and you need to buy something else.
In that case, there IS hope. There are a few methods of handling this, but my favorite method is to put your second router in a DMZ of your first.
The first step is to log in to your ISP provided modem/router manually and look around in there to see if you can find a DMZ, Static NAT, or Default Server section. Different routers have different names for their DMZ section.
Now, enable a DMZ, Static NAT, or Default Server to point at your your second router's WAN or External IP address. (The one that we found earlier and you should have written down.)
This should allow traffic to flow directly through the first modem/router to the second one.
From there, follow the normal port forwarding procedure on your second router and you should be good to go.
It's important to note, that both routers that you may be running might also run additional firewalls.
If you are looking to port forward, you are going to need to disable your routers' firewalls, if they have them, to get your ports to test forwarded.
There are also other methods for getting around the double router problem including converting your second router into a WAP or switch.
You should be able to "convert" the new router into the correct device by disabling the router function and using it as just an ethernet switch or wireless access point.
We have a guide to doing that conversion process for most types of routers here:
Using a Second router as a Switch/Hub/WAP
If you have a fairly new model Belkin router, check this guide for a possible easier fix for the problem:
Putting a Belkin router into Access Point mode
Once you've successfully converted the new router to be used as just an ethernet switch or wireless access point, all your port forwarding configuration will be done in the FIRST unit, your original "combination Modem/Router".
The new router will no longer block anything and will simply pass along the signal it gets from the first router.
One last note:
Make sure that you either use the DMZ method, or the conversion method, but do not confuse the two.
I hope this helps.