Understanding Port Forwarding – Beginners Guide

Firewall-function-overviewWhat is Port forwarding ? and What Does it do?

Port forwarding is a technique that is used to allow external devices access to computers services on private networks.

It does this by mapping an external port to an internal IP address and port.

Most online gaming Applications will require you to configure port forwarding on your home router.

To understand port forwarding you need to understand what a TCP/IP port is and how ports and IP addresses are used together.

You will also need to appreciate the difference between internal and external IP addresses and internal and external ports.



TCP/IP Ports

A TCP/UDP port identifies an application or service on a machine in a TCP/IP network.

On a TCP/IP network every device must have an IP address.

The IP address identifies the device.

However a device can run multiple applications/services.

The port identifies the application/service running on the machine.

The use of ports allow computers/devices to run multiple services/applications.

Standard Port numbers are allocated to server services (0-1023) by the Internet Assigned Numbers Authority (IANA). e.g Web servers normally use port 80 and SMTP servers use port 25.

The combination of IP address plus port is known as a socket. See Understanding TCP/IP Ports and Sockets

As an example. Imagine sitting on your PC at home, and you have two browser windows open. One looking at the Google website and the other at the Yahoo website.

the connection to Google would be:

Your PC – IP1+port 2020 ——– Google IP2 +port 80 (standard port)

the connection to Yahoo would be:

our PC – IP1+port 2040 ——–Yahoo IP3 +port 80 (standard port)

Notes: IP1 is the IP address of your PC. Client port numbers are dynamically assigned and can be reused once the session is closed.

Returning to Port forwarding..

On home or small office networks the  router uses NAT (Network Address Translation) which allows internal devices to share a single external IP4 Address.

The IP addresses on the Internal network are private addresses and are not routable on the Internet.

External computers or devices only see the public IP address that is assigned to the NAT router Interface.internal-external-IP-addresses

The NAT router maps an Internal IP address + Internal Port to the external IP address + external port.

External devices send packets to the external IP address and port.

The NAT router maps those packets and re-transmits those packets on the Internal network to the Internal IP address and internal port.

The ports used by NAT are normally randomly assigned which is OK when the session is initiated from the Internal network.

However if you want, for example, to host a website on your internal network and that website needs to be accessible to external clients then you will need to use a standard port (port 80 for http) as the external client expects this.

To do this you statically map the external IP address + port 80 to the Internal IP address of the web server + port 80.– This is port forwarding.

For home users the most common reason to use port forwarding is gaming.

Enabling Port forwarding and Checking Open Ports

Before you setup port forwarding you will need to configure a static IP address for the Internal device.

This step is important as the forwarding will be set to send packets to a specific internal IP address.

Depending on your Application you may need a list of ports that need to be available from the the external network (i.e. Internet) and forwarded to the internal network.

To configure port forwarding on your router you will need Admin privileges.

This site has a comprehensive guide covering 100s of routers, and also port lists for many of the games/applications.

Regardless of exactly how you configure it, as it varies by device, what you are essentially doing is creating a mapping table that maps an external address and port to an internal address and port.

This video shows how to configure port forwarding on a BT Home Hub.

This video shows you how to set it up on a Linksys router. It also shows you how to set a static IP address for your machine.

Once you have forwarded the ports you may want to check that they are really open using an open port checker.

Connecting to a Forwarded Port

To connect to the forwarded port from the Internet you will need to know the external IP address of the Router and the Port number that has been forwarded.

However using an IP address instead of a domain name is not very convenient, in addition the external IP address can change as most ISPs assign these addresses using DHCP.

Therefore when using port forwarding you might also what to consider using Dynamic DNS.

Port Forwarding Example

Below is a screen shot of my home router configuration which shows the ports I’ve forwarded.

Notice my router doesn’t have a field for the external IP address as it isn’t really necessary.

However some do and it is usually then configured to 0.0.0.0 .

port-forwarding-example

Checking Open Ports

You can see from the screenshot above that I’ve opened ports 1800 and 1884 and 8884.

I used the online open port check to check those ports and also one that shouldn’t be open and you can see the results below.

open-port-checker

Note: I’ve hidden my external IP address for security reasons.

Port Triggering

This is a type of dynamic port forwarding were ports are opened by an application, and don’t require prior setup.

The application connects to the designated trigger port on the router which tells the router to open an specific port for inbound traffic.

After the application has finished it closes the ports it opened.

Because the ports aren’t left open it is considered more secure than port forwarding.

UPnP ( Universal Plug and Play)

You may have noticed that your games work OK on the Internet even though you have not configured port forwarding.

This is most probably because the router as UPnP enabled.

Using the UPnP protocol an application can open ports on the router.

The screen shot below is taken from my router and shows a standard port forwarding rule and one setup automatically using UPnP.

router-upnp

Using UPnP is considered insecure and to be avoided if possible. However most routers come with UPnP enabled by default.

Summary

Port forwarding Maps external IP addresses and ports to Internal IP addresses and ports allowing access to internal services from the Internet.

It is configured on home routers and it is necessary because home routers use NAT which isolates the home network from the Internet.

Common Questions and Answers

Q- Is the External IP address mapped to the Internal IP Address?

A- No the external port is mapped, and not the external IP address. the external IP address might change see Dynamic DNS

Q- Should I use a static Internal IP address or can I use addresses assigned by DHCP?

A- You should always use a static one.

Q- Do I need to forward both the TCP and UDP ports?

A- It depends on the application. You need to check which ports the application uses.

Q- How do I know if my device has a static address or a dynamic one?

A- You have to go to the device and examine the settings.

Q- How do I know what port I need to forward?

A- You need to know what port the service you want to use is using.However most home routers will have a list of common games and applications and you just need to select it and it will automatically select the ports.

select-game-forward

Q- How do I know if I have configured it correctly

A- You can use an online port forwarding checker to check that the ports are open.

Q- What is strict NAT?

A- Microsoft define three levels of NAT- Strict,Moderate and Open. Devices that perform strict or moderate can affect Gamers on Xbox. See this article for help

Q- Does port forwarding affect my home network security?

A- Yes because you are exposing the home network to the Internet

Related Tutorials:

References:

Please Let me Know if you found it Useful
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48 comments

  1. Hi Steve,

    A few questions related to Ihsan Nurul Iman’s original question:

    1. If the other person who wants to play the same game is connected to the same router as him, then would they both share the same external IP address?
    2. Could you explain how the mapping in your reply (191.168.1.5:50000 66.66.66.66:10000) would also work? If i hosted a private game server on my computer with the mentioned port forwarding, would people who are trying to connect to my server have to type in the IP address along with the port number?

    Thanks in advance! Hope my question makes sense as I’m new to this.

    1. Question 1- All computers on the internal network share the same external IP address.
      Question2 – yes they always need an IP and port. The IP would be the external Ip and the port whatever you have mapped. If we take two web severs located on our network both use port 80 internally.
      On the router I would map port 80 to 192.168.1.21:80 and port 8080 to 192.168.1.21:80

      and external client would connect to external_ip:80 to connect to the web server on 192.168.1.21 and external_ip:8080 to the web server on 192.168.1.22

      Does that make sense?

  2. Hi Steve,

    first thanks for your fantastic explanation. But I got some doubts after learning port forwarding. I’ve gone to several references and didn’t find any explanation I’m really looking. I hope you’re willing to tackle my doubts:

    1. Where’s our public IP address? Is it on our home, or our ISP’s office?
    2. Let’s say my public IP is 66.66.66.66 and my device has a private IP of 191.168.1.5. I want to forward 50000 port for playing X game. So if the game requests for 66.66.66.66:50000, it’ll go to my private IP right? Now what would happen when another person who’s also using the same public IP as me, also want to play the same game, he want to forward 50000 port? How the router handle that? Because I’ve set the 66.66.66.66:50000 to go to my local, not his/her.

    Thanks in advance!

    1. The public Ip is on your router. You router has 2 IP addresses see here
      https://stevessmarthomeguide.com/internal-external-ip-addresses/

      Your public IP address may change but no one else has it at the same time as you.
      As long as you have the ip address 66.66.66.66 no one else can be assigned it.

      2. Let’s say my public IP is 66.66.66.66 and my device has a private IP of 191.168.1.5. I want to forward 50000 port for playing X game. So if the game requests for 66.66.66.66:50000, it’ll go to my private IP right? – Correct but the ports don’t have to be the same e.g you could use port 10000 on the external IP address. It ia a mapping 191.168.1.5:50000 <=> 66.66.66.66:50000
      191.168.1.5:50000 <=> 66.66.66.66:10000 would also work.
      Hope that helps
      Rgds
      Steve

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