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We see many questions in the forums regarding port forwarding, so I have put together a primer to try to help those who are new to the concept.
WARNING: Port Forwarding exposes devices on your LAN to the Internet. If you DO NOT NEED to port forward, DON'T. This guide tells you how to and why you might need to port forward. The reader assumes any and all responsibility for any damage of, or intrusions into their network caused by port forwarding.
What is Port Forwarding?
Port forwarding is a mechanism used in IPv4 to allow a computer, smartphone, or other device (the SOURCE) outside your Comcast HSI connection to connect to a device on your LAN (the DESTINATION). A typical Comcast residential user network might look something like this:
In this diagram, a Cable subscriber (you) has a webcam, NAS, or media server on the internal LAN behind his router/gateway. It is configured to listen on TCP port 8080, in this case. Other devices or game servers that you have may be configured to listen on a different (or many different) ports, but the principle is the same.
Let's say that the subscriber (you) want to make that server available to a family member in a different location, so they can see your webcam, or look at pictures on your media server or NAS. In order to do that, your family member needs to be able to connect, often using a web browser, or a smartphone app. The problem is that your web server is on your LAN, protected by the firewall in your router, AND it has a private address, which cannot be used on the internet. The private subnets you will see most commonly are the 192.168.0.0 and 10.0.0.0 subnets. Subnetting is beyond the scope of this discussion, as most home routers are set up out-of-the-box to use one of these submets. Under normal circumstances, you will probably never have to change this. The problem lies in the fact that these subnets cannot be routed over the internet. Port Forwarding is intended to solve that problem.
As seen in the diagram, your home router (and you must have a router of some type to perform port forwarding) is provided with an address on the public internet by Comcast. Because of a shortage of IPv4 addresses, home routers use Network Address Translation (NAT) to allow you to connect a large number of devices to your router, and give them the ability to connect to other devices on the internet. This is done by assigning each requested connection from your LAN a TCP port, so the router can track the connection. The combination of the IP address and port number is called a socket. So, when you use your computer on your LAN to go to www.google.com, your computer (the source) sends a request using a random port (usually above port 1024) to the destination (www.google.com) on port 80 (the standard http port. Because of NAT, the Google server sees your source address as the WAN IP of your router. How, you may ask. is this related to Port Forwarding? Well, port forwarding is essentially the same process, but in reverse. Think of it as inbound NAT. You router has a public IP address, but by default, it doesn't listen on many ports. This is for security, so that someone on the internet cannot easily get on to your network. Now, you have a media server or an IP Camera that you want Granny to see, so you have to tell your router to listen on a port so that you can give Granny a link to it that she can put in her web browser (how she does that we will discuss later).
How do I set up Port Forwarding
First, you have to set up your router. There is an excellent website at http://portforward.com, which will walk you through the steps of how to forward ports on just about every known router, so I am not going to go into detail on any particular model. Suffice to say that when you set up port forwarding, you tell your router to listen on a particular port (in the case of the diagram, it is port 8080), and you also tell it where to send that traffic, when it sees it. In this case, the router is told to send all traffic it ses incoming on port 8080 to the internal device at 192.168.1.200. Below is an example of the screen for configuring a Custom port forwarding service on a Netgear WNDR3700. Other router screens will look different. This is just one example:
OK, so I have set it up...how does Granny get there...
Granny has a computer with a web browser. Lets say you are letting her see your IP Camera. instead of port 80, your IP Camera is designed to listen on port 8080. Under normal circumstances, you would forward the same port externally as the device listens on, so you would set up your port forwarding to listen on the WAN interface on port 8080, and internally, send all port 8080 traffic to the IP address of your IP camera. Now, when Granny browses google.com, she just types inwww.google.com in her browser, and it goes there, right? That is because google.com is listening on a well-known port for http traffic (port 80), and browsers automatically know that you want to go to port 80. What they don't know is that your router is waiting to send traffic to your IP Camera on port 8080, so when you tell Granny how to get to your camera, if you are using any port other that 80, you MUST specify the port, and that you are using the http protocol. So, you would tell granny to put the following in her browser address bar:
To find the WAN IP of your router, you can either look at the Status page in your router interface, or browse tohttp://whatismyip.com
If you don't want to give her an IP address, you would need to use some type of Dynamic DNS service (not within the scope of this discussion) to translate your WAN IP into a hostname, but you still need to specify http and the port number, like this:
http://myipcam.somedomain.org:2000 (the actual name will depend on your Dynamic DNS provider)
What else can I do with Port Forwarding?
The principles are the same for pretty much any device or server that you want to make available to sources outside your home. You can port forward Windows Remote Desktop Protocol, so you can log into your PC from another device with an RDP client. You can run a web server (although publicly accessible webservers are technically against the Comcast AUP for residential connections), you can access your own media server from your smartphone, so you can listen to your music wherever you are...the possibilities are pretty much endless, BUT make sure that you secure the devices you are allowing access to with strong passwords. While many security experts frown upon the concept of 'Security by Obscurity' I personally don't see that it hurts to change the port you are using for some services, especially the more common ones...Any hacker knows that Windows Remote Desktop Protocol runs on port 3389, so instead of setting your port forwarding up to listen on port 3389 on the WAN IP, use a different port (above 1024 is recommended. The highest you can go is 65535). You can still tell the router to forward the traffic to port 3389, so you don't have to mess around with the registry settings for your RDP setup on your Windows machine.
That is basic port forwarding in brief. If you have any questions, please post it in the forums in the Home Networking / Router / & WiFi Gateway Help board and we will try to help.
Be aware that as of writing this (April 2013) there appear to be some issues with port forwarding on some of the Comcast supplied gateway devices. Unfortunately, there isn't much we can do to remedy those, if you have set up port forwarding properly. The suggested solution is to have the gateway placed in bridge mode, and buy your own router to do your port forwarding.
Also there are some quirks to setting up port forwarding on the SBG6580 gateway. See this post for details: