Multiplayer problems since 0.12.4

[Doctor Alpaca]

A friend and I have been playing multiplayer rounds for a while now without problems, but now we updated the game and I have a new router. He now gets a "Couldn't establish connection to the server." messages. I first thought it might be the new router so we tried connecting via Hamachi but that yields the same result. I also tried starting a completely fresh Factorio install as the server, same problem. So now I'm unsure whether the problems are because of the new version or because of my new networking situation. Anything else we can try?

[Cyberdragon]

It's not you, the game does not use the internet properly. I've posted a topic on this in bug reports, they are using the wrong internet protocol.

[SirRichie]

It's not you, the game does not use the internet properly. I've posted a topic on this in bug reports, they are using the wrong internet protocol.

I am curious what the "wrong" internet protocol is.

Back to the original question: In 12.4, the networking mechanism now supports client-server in addition to peer-to-peer. If you want the same behavior as in 12.3, make sure peer-to-peer is enabled in the options.

Also, new routers can cause issues. It does not mean that the router is bad, just that Factorio uses a protocol/mechanism that does not work with this specific router.
Hamachi may not be the most reliable tool to test with. The reason is that Hamachi installs an additional network adapter in Windows (which is fine) and that it now depends on your order of network adapters in Windows. Many games (including AAA titles) only try connecting through the first adapter in the list. If that is Hamachi, you can play via Hamachi. If that is your normal Ethernet/WiFi adapter, the game will not be able to connect through Hamachi.

Side note: You have enabled port forwarding in your new router?