In order to play old siege with other people, we need to emulate a local network (LAN). To do this we use an application called Radmin VPN.
Note that there is no official matchmaking in old versions of siege, since there are no servers to host them. Therefore only custom games or PVE gamemodes can be played.

Setting up Radmin VPN

1. Download the Radmin VPN installer from their website here and run the installer.

2. Once the installer is complete, and the Radmin VPN window is open, you will need to configure a setting.
   Go to System → Firewall exceptions in the Radmin VPN window and press Allow all apps.

3. If the big power button in the Radmin VPN window is red, indicating that Radmin VPN is disabled, go ahead and press it to enable Radmin VPN.

Now Radmin VPN is configured and ready for use.

Creating or joining a Radmin VPN network

In order to play with other people you first need to be in the same Radmin VPN network as them. You can either create your very own network that people can join, or you can join an already existing network. Creating or joining a network has no effect on who has the ability to host. For example, person 1 can be the owner of the Radmin VPN network while person 2 hosts the custom game without any issues. The important part is that person 1 and person 2 are in the same Radmin VPN network.

Creating your own private Radmin VPN network

To create your own Radmin VPN network you do the following:

1. At the top of the Radmin VPN window, navigate to Network → Create Network and press it.

2. Input your desired network name and password.

Now you will be able to see your network in the network list below.

Joining an existing Radmin VPN network

To join an already existing Radmin VPN network you do the following:

1. At the top of the Radmin VPN window, navigate to Network → Join Network and press it.

2. Input the network name and password for the network you wish to join.

Now you will be able to see the network you just joined in the network list below.

Hosting or joining an old siege custom game

Now that you have Radmin VPN set up and you and all of the other players are in the same network, it is time to play. But how does one do that?
In order to play, there needs to be one person that hosts the game, and all others join it.

Does it matter who hosts the game?

Yes, it can matter in the circumstance that the individuals participating are located far apart from each other geographically. The individual hosting the game will act as the server. This means that based on how far away the other players are from the host, they might have high and sometimes uncomfortable game latencies and experience network lag.

As an example, suppose three people play together. One is in the United States, one is in the United Kingdom and one is in Russia, the person located in the United Kingdom shall preferrably act as the host, because that individual is geographically right between the other two individuals. This makes sure that the game network latency, or 'ping', is as low as possible for everyone participating.

How do I host a custom game?

In order to host a custom game, you can follow these steps:

1. In the Rainbow Six: Siege client, navigate to Play → Custom Game → Create Local. Note that this may differ slightly depending on which version is played.

2. Configure a new playlist or select an already created playlist. This is where you choose which map, gamemode and other game rules the custom game shall have.

3. Press Create Custom Game.

4. You should now see a lobby with only you in it!
   Now other people that are in a Radmin VPN network with you and are running the same version of the game as you can join your custom game as specified below.

How do I join a custom game?

In order to join an already hosted custom game, you can follow these steps:

1. In the Rainbow Six: Siege client, navigate to Play → Custom Game → Join Local. Note that this may differ slightly depending on which version is played.

2. Look for the game host's username in the list. If you cannot find it, refer to the Why can I not find the game host's username in the join list? on the frequently asked questions page. // EDIT LINK

3. Press the game host's username in the list and press Join as Player.

4. You should now see a lobby with the other players in!