Disclaimer: Because this Guide heavily depends on the software AND hardware you are using, I won't go into the details.
Google or any other Search-Engine is your best friend!
In order to allow requests to reach your computer we need to configure 2 Components:
Configuring your Local Firewall
Your Firewall is what blocks unallowed requests from being processed and/or answered from your Computer.
This is a good thing, but in this case we need to setup a special rule for it in order to allow TripleA to be accessed by other clients over the web.
On Windows it's often sufficient to just allow a single Application to communicate through the Firewall. You can add TripleA to the "Allowed Applications" in your Control Panel, if you are not using the installer, you need to add java instead.
On all operating systems you have the possibility to open single ports manually. TripleA uses port 3300 by default.
DO NOT TURN OFF YOUR FIREWALL FOR A LONGER PERIOD OF TIME UNDER ANY CIRCUMSTANCES
Finding your Firewall-Settings
Open your Control Panel, change the View By setting to "Small/Large Icons" and click on "Windows Firewall">Allow a program or feature through Windows Firewall
Based on the distribution you are using this will differ. On Debian based systems (including Ubuntu) you can use ufw (uncomplicated firewall) instead of manually messing with iptables. With ufw you just enter
sudo ufw allow 3300 in a terminal. Done.
If you are using Linux you probably know how to figure out how to do this on other distributions. There are just too many to list them all.
I have no idea... (If you are a mac user, please tell me so I can edit this in here later)
Configuring your Router
Assuming yor Computer is not connected directly to the internet, but connected to a router instead which is most likely the case, we need to tell your Route it's supposed to route all TripleA-related traffic to your computer.
This is called "Port-Forwarding". You can find tons of guides for every router model out there in the Internet.
It would be impossible to list all of those here!
Make sure your Router forwards Port 3300 to Port 3300 on your local machine.
If you are asked for the Protocol select TCP.
Computers often don't work like you want them to, here are some tricks that might come in handy if something is not working for you:
Try temporarily disabling your Firewall. If everything is working fine with your Firewall disabled, you haven't setup your Firewall correctly. If it's still not working your Router isn't correctly forwarding the traffic to your PC.
canyouseeme.org is a website that helps you checking if your configuration is working. Just enter the port you want to check (in our case 3300).
If you still have any questions, feel free to ask!
added link https://forums.triplea-game.org/assets/uploads/files/1523253787960-upnp.zip