I'm not really knowledgable on the subject of networks.
I'm wondering if there is any sort of application that allows you to play LAN games via the Internet, but doesn't require connection to external servers, like Hamachi, GameRanger or Tunngle does?

I'm talking about a tool functioning similarly to popular LAN emulators for Warcraft 3 which allow you to host and see LAN games of/for others after direct connecting to one of them, who becomes the host. Is there something like this, but universal, that is designed not only for a single game?

Assuming you're playing with friends and the game isn't too network intensive, you could set up a VPN at one of the sites, have all players connect to it, and have it treated as a LAN. Or, at least, this *should* work.

(I haven't actually tried it, but then again, I don't play multiplayer games.)

But these do require external servers, don't they? I'm looking for a tool that is independent of external servers and allows to create virtual LAN by simply direct connecting to each other.

A VPN is one machine that accepts incoming connections, assigns an "internal" IP address, and allows all computers in the internal network to communicate with each other. Said machine would require two network interfaces to do said job, but some high end routers could do that thing.
Yes, you will need to connect to the router that does the VPN, which may be the one you yourself have and use, not sure if that counts as an external server or not.

Awesome, looks like that's exactly what I need! Thanks. ;)

There's no requirement for additional network interfaces to do this. Anyway, OpenVPN also works for this. You would need to pass through the ports to the internal machine running OpenVPN.

Wouldn't the machine need to communicate with two different subnets though? Can one network interface be part of two subnets?

On Linux, the VPN appears to applications as a separate network device. There's the physical device, which the packets are sent through encrypted, and there's the VPN device, which is a virtual device; packets sent there are encrypted and then sent through the physical device.

Linux actually lets you create lots of virtual devices. You can even create network namespaces and veth devices to communicate between them, allowing you to run applications as if they were on separate machines from a networking perspective.

Yes, it can. E.G. you decide to place all your wireless APs in the 172.18.153.0/24 range, but by default they all start with 192.168.1.1/24. You can configure your NIC both as 192.168.1.249/24 and 172.18.153.8/24 in addition to it's primary IP/network. Keep in mind for routing purposes all configured networks are considered directly attached.

Hah, thank you. This would have been extremely useful a few years back, but still good to know.

You can set up a point to point VPN between two computers (or two routers) using just the software that comes with the OS without installing any 3rd party applications at all if you desire. Both Windows and Linux come with built in IPSec tunnel capabilities and all one has to do is configure both ends of the tunnel to connect two computers over the Internet without any 3rd party services being involved.

The only real problem is that the software that comes with the OS to set this up is just your basic dumb control panel type applet, without an easy-button. Additionally, it is up to the person setting up the OS provided VPN to understand how firewalls and NAT work, and to make sure their firewalls are configured properly to allow the proper protocols and ports out of their network, and to allow incoming protocols and ports into their network including any protocol and/or port forwarding that is needed. The average computer user and even many people who are administrators generally do not have the low-level networking knowledge and experience to set this all up, and without some kind of "wizard" type software to do all the details magically including things like NAT traversal, it can be complicated to learn how to set it all up and get it to work.

Plus, the person on each end of the VPN must learn the ropes on how to set it all up which further complicates things, and if you want to set up a link between more than 2 end points, you then have to decide on either a client-server model where one person is a central hub for all the others, or if it is a completely peer-to-peer mesh type topology (such as what apps like Hamachi offer). Then one has to know how to configure all that completely manually by hand.

Since that's generally a huge complex pain in the ass, most people use 3rd party VPN software even if they are experienced and knowledgeable about these things because they're just easier to use and less fraught with problems that need manual troubleshooting and resolution.

That being said, even though many of the VPN products out there such as Hamachi have NAT traversal techniques built into them so users do not need to be firewall ninjas, quite often relying on the built in NAT traversal techniques to make a VPN work often leaves the performance of the VPN less than desired compared to manually configuring the border router's firewalls and port forwarding. So even when using such software it's recommended to learn what ports and protocols the software prefers to use and twiddle the router firewall (and client firewalls if need be) to work optimally. I can say from personal experience with Hamachi for example that manually setting up the firewall properly gives much better results than relying on their built in NAT traversal tricks, and the same is most likely true for other software as well. I can also say however that manually setting up the firewall for best results with Hamachi is a rather complex thing to do due to the plethora of external machines it contacts over both TCP and UDP on a huge number of ports that all serve different purposes. It's not for the faint of heart to say the least.

So, even though the OS has built in VPN support that can be made to work just fine, the majority of people out there at every skill level are generally better served by easy-button solutions like Hamachi, Tunngle or other such software whether it is open source, or proprietary, free, or for cost. The only reason some of these solutions actually use central servers for things is in order to provide that "easy-button" solution to make configuration simpler, as this allows them to more easily solve the NAT traversal problem. In that regard, these programs are quite similar to software like Skype which has done similar "magic firewall bypass" tricks for well over a decade now, to make life easy on their users rather than requiring them to become Internet RFC editors to get their software rocking and rolling. :oP

That's some of my thoughts to share anyhow. :)