Sunday, November 21, 2010

Port forwarding through a modem and router to expose and enable communications to local services - How to?

I’ve been trying to expose few ports on my machine to the public, for the past few days. I started off assuming that this will be a simple task to do. Even after trying different configuration settings found from Googling, I was not successful. As I’m not a networking guy, I was missing few basic things which were crucial for this to work. Combining all these learning, here are the steps that you will have to take to successfully forward ports to your desired local machine.

The purpose:
This might vary from person to person; sometimes you might want to locally deploy a web site and expose it for testing to your friends OR you might want to open up your MySQL or utorrent Web UI so that it is publicly accessible. From a networking perspective, all of these scenarios lands up in opening up ports in your router to the PC in which these services resides. By default, most of these ports are closed.

Network Structure:
Assuming that the following is your existing network structure, let’s see the steps involved.

Initial Steps:
Here are basic information needed to start with
  • The port to be opened up; in my case, this is 61799
  • The local IP address of the system which will be running the target service; Ex:
  • IP address of the router and your public IP; the IP address that represents you in the world.
Make sure that the IP addresses are static.
Before starting with the steps involved, we need to make sure that all these addresses does not get altered over time. This is important because most routers assigns IP addresses dynamically to each of the PC/clients using DHCP, as and when they initialize a connection with the router. Based on the Lease time (the time for next renewal), the IP assigned to one client-PC might get changed later.

So, let’s first give your PC a static IP first. if your PC is already on static IP, skip this step.
Conceptually, what you are going to do is to understand the preferred network settings and set them to your network adapter-card. You will also disable dynamic IP address renewal in your PC. Based on the OS that you have, static IP assignment process can vary. Here is one site that explains this. A typical settings can look like this.

One most important thing here is that, even though you have told your PC to have a static address, you should tell your router you assign the same. Otherwise router will still try to dynamically renew the PC’s IP which will lead to conflicts. To accomplish this, set the router in such a way that the router’s DHCP does not include this address. However, care should be taken that the existing clients can still run on dyncamic IP assignment process. In order to do this, the best step is to set a range for DHCP an assign an IP out of this range to the PC. It is always better to limit from the top range so that you are not touching the router/modem IP settings, which are usually on the lower range. From my settings below, i freed up IP addresses above

So, now the
  • Local PC is set with the IP
  • The port to be opened up is 61799
  • The router IP is (usually this is this is the IP at which the router’s web console resides. 
Port forwarding:
Now, let’s welcome and guide the requests from the outside world to the local PC. This is done using port forwarding. For beginners, let’s assume that port forwarding is like guiding a guest from one door to the destination room in your house. Some time, this can involve going through multiple doors(ports) OR just a single door. This is a very limited or primitive way of looking at it. From networking perspective, there is more to it. Just like your house to be secure, most of these doors are closed, by default. That’s why you need to open this up.

Port forwarding is sometimes called Virtual Servers. The trick is to access the router’s configuration page and find the page for this setting. Basic information, the page will ask for is
  • the service name (just for your identification)
  • the incoming port (when some one comes to this door)
  • the local PC (route to which computer)
  • the target port (to which door; the port at which your service is keep listening to so that it can accept the request). 
Here are the screenshots as per my router.

Allow access for the Port in Local PC - Firewall changes:
The final step is to make sure that the port is opened for access in your local PC. To open the port in your local PC, access the advanced settings of your firewall in your local PC and add a port exception. Most windows OS handles this section seamlessly. Basically we are trying to tell the firewall that if any request comes through 61799 door, let it come through.if you are looking for Step-by-step instruction for this, googling will help! Below are the major parts in this process, in screenshots.

Public IP Address:
Let’s identify the public IP of your’s . Most of the ISP( internet service provider) will assign an IP for your network, each time you connect to it. if you have a static IP bought from your ISP, ignore this section. Otherwise, you can have to find out your piblic IP using websites like

That's it. You’re done. Use any of the online port check tool to confirm whether the port is open. is one such site. Make sure that your service is running at your local PC while checking. Once this is confirmed, you should be able to access the service at [public IP]:[port]. For example, an http service can be requested by http://ppp.ppp.ppp.ppp:61799/ where ppp.ppp.ppp.ppp is your public IP.

There are FREE tools available that can give you a domain name for your IP, even if changes dynamically. You can register with them that will get you and address like You might have to download few apps to refresh thier records as and when your IP changes. These changes will be done automatically and you will now have a public address by using this. With this, your address for the service will be Few free sites are, (free for ASUS products)

Multiple Router Scenario:
For most common network structures, the process is now complete. But tragically, my network structure were slightly different. It is something like this.

Here, the modem that i was having, the one that my ISP gave me, was not really a modem; that was also a router. In this case, i had two routers in my network and all the clients were connected to my second router.

The ideal way is to make the second router work as a Access Point and make the above explained changes in the primary router. This is possible, if your secondary router is capable to work as an Access Point. Even though my router ASUS RT-N14U is capable to do so, i tried to make it work without any changes to the existing system, keeping the secondary router as a router itself.

Primary router changes - Port Forwarding to secondary router:
Here are the steps involved to make this happen:
Connect the primary router directly to your local PC.
Access the administrative module which might be residing at In my case this was a Beetel 220bx modem. so, by default, it will not show you the advanced options. Little bit of googling showed that and are the two pages that exposes the options. Accessing main.html provided me the options to configure a virtual server in my primary modem.

The virtual server in my primary modem was set as
  • Incoming port: 61799
  • Target IP: IP address of the secondary router (
  • Target port: 61799
What i told here is to route all the requests from public at 61799 to the same port at 61799 to my second router, which is already set to route the request to my local PC.

Once the settings are done, turn off the machines and get back to your original network structure. This was the final step i have to do and get this working.

If there are other ways to achieve the same, let me know as comments.