ELI5: Port forwarding

Once upon a time, there was a very, very long street. It was so long that you could build over 4 billion houses along this street. The people who designed and built this street were very smart, and they thought that this would be enough for everyone to have a house on the street.

This street was called Fourth Street.

The people who lived on Fourth Street never left their house, but they loved sending letters to each other. As long as you knew the address of someone else living on Fourth Street, you could get a letter delivered to any house on the street very very quickly.

Every house on Fourth Street had a very special letterbox. The letterbox had many, many little compartments in it. In fact, each letterbox had 65,535 little compartments. These were called ports.

People would use these ports to manage their mail. For example, a person might want all of the letters related to Minecraft to arrive in one particular port. So, in order to send a letter to someone, you had to specify a port in addition to the address.

At first, there weren’t many people living on Fourth Street. But it grew in popularity very quickly. Everybody wanted a house of their own, with their own letterbox that they could use to exchange letters with everyone else. Within 10 years, over half of the street was occupied.

In another 5 years, the entire street was completely filled.

We need taller buildings

There were still more and more people that wanted to move into Fourth Street. It was decided that instead of giving a house to every single person, multiple people would have to share a house. The houses on Fourth Street started to be converted into taller apartments, where many people could live at the same address.

Each apartment was still only allowed to have a single special letterbox on the street. But the people living on Fourth Street loved the special letterboxes so much that they would install one right outside of their room door.

Even so, the postmen could only deliver letters to the shared letterbox on the street. They didn’t know how to navigate the insides of the apartment.

To solve this problem, each apartment would have a doorman who would check the shared mailbox for letters, and deliver them to the letterboxes inside the apartment.

But how did the doorman know where each letter should go? The doorman had a little rulebook that had instructions like:

Letters in port 80 should be delivered to port 9000 in the third room’s letterbox.

If you wanted to talk to someone about Minecraft, you could just tell them to send letters to your street address on port 100, and then tell your doorman to forward all letters in port 100 to port 200 in your personal letterbox.

And so, everyone was able to share the addresses on Fourth Street and send letters to each other again.

What’s next?

Meanwhile, there is a new street being built. This one is even longer than Fourth Street. It is incredibly, mind-bogglingly long. This new street can fit far more than a measly 4 billion addresses. It can fit 340,282, 366,920,938,463,463,374,607,431,768,211,456 (340 undecillion) addresses. Surely, this time, we will never ever run out addresses again.

This new street is called Sixth Street.

So… what’s port forwarding?

In real computing terms, Fourth Street is the IPV4 address space. There are truly over 4 billion IPV4 addresses (although not all are reserved for use as public IP addresses), and we have truly run out since 2011.

Computers also communicate via ports. Instead of letters, they send packets. There are certain standard ports that everyone has agreed to use for certain purposes. For example:

• Port 80: default port for HTTP web traffic
• Port 443: default port for HTTPS encrypted web traffic
• Port 25565: default port for hosting Minecraft servers
• etc

But you can really use any port you like - there’s no difference.

The street-level address from the analogy would be your public IP address, and it might look something like 172.217.167.78. This part of the network is called the Wider Area Network (WAN).

Within the apartment we define the Local Area Network (LAN), and each room would have a local IP address which might look something like 192.168.1.1.

The doorman described in the Fourth Street analogy would be a router in computing terms, and the job they are performing is called Network Address Translation (NAT).

The router is the bridge between the WAN and the LAN.

The little rulebook described in the analogy are the port forwarding rules that you can configure on your router. These rules simply tell your router how to route external WAN traffic on a particular port to an internal LAN address on a particular port.

It’s common for the external port to be the same as the internal one (eg, external port 25565 to internal 192.168.1.10:25565), but there is absolutely no requirement for that to be the case.

What happened to Fifth Street?

IPV5 was an experimental protocol that never took off. Lifewire provides a good overview of what happened to it.

Double NAT

Just a note - the most common problem you’ll run into if you can’t get port forwarding to work is when your network is set up so that there are two layers of routers performing NAT. This is called double NAT.

Unless you have very special requirements, there is almost never a good reason to have more than one router performing NAT in your home network.

You could probably port forward from router 1 to router 2, then from router 2 to your computer. But the better solution is to restructure your network so that you don’t have double NAT.

Hopefully, you have a better understanding of how it all works and are better equipped to debug your home network now :)