Yup it's peer to peer. You can play on LAN without Steam
but, you'll need Steam working in the background if you wanna play with friends over the internet
If you're talking about dedicated servers then no. This game was primarily made for P2P.
If you start a game normally, it will create a session, but the problem is that routers will block the port. So, you'll only be able to play it on LAN.
But if you use Steam, it will allow you to host a public game.