Symmetric NAT sucks! They prevent peer-to-peer connections!

Introduction

When you go to www.google.com from a computer, the computer sends a request through a Network Address Translation (NAT) before hitting www.google.com. This NAT translates the private IP address of the computer to the public IP address of the NAT. Additionally if the NAT is symmetric, the port number of the computer is translated to a new port number.

So how do you check if you are behind a Symmetric NAT?

Steps

the following link returns the IP address and port number of the NAT

• http://util.marcuschiu.com/api/my-ip-address-and-port

we need to check whether the NAT keeps the same port number of the computer

we shall use curl to enforce the outbound port of the computer

curl http://util.marcuschiu.com/api/my-ip-address-and-port --local-port 5555

if the returned port number is DIFFERENT then your computer is behind a Symmetric NAT!

Not Behind Symmetric NAT

❯ curl http://util.marcuschiu.com/api/my-ip-address-and-port --local-port 5555
{"ipAddress":"17.23.45.4","port":"5555"}

Behind Symmetric NAT

❯ curl http://util.marcuschiu.com/api/my-ip-address-and-port --local-port 5555
{"ipAddress":"17.23.45.4","port":"4535"}