Offline LAN party with Steam games

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Me and my friends are thinking about a lanparty, where 90% will be played Company of Heroes 2 and Counter Strike:GO (so both Steam games that we all bought).
But the issue is, that the best location for the LAN party is without internet connection. ... Well yes, there is mobile network with some reaaaaaally slooooow internet connection (hardly to by used when checking for emails on smartphone), but I would rather put it, that there is no connection.

So the question is. Is there any way, that 4 steam friends can connect only via local network, not using steam internet login at all ? And after that play local LAN versus each other or bots ?

Or maybe the first user verification and steam login can be done over internet, but than the connection within the matches can be only LAN.

[Coz takhle dat si spenat ?]

Please, anyone have any idea ?

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Really no clue on this issue ? :(

[Tev]

I'm pretty sure it'll work.

Ultimately it comes down to the game, but

If you've used your Steam -account within the last 30 days on your PC, you can enter offline mode by unplugging the RJ-45 (Ethernet)/switching Wi-Fi off when you're launching the Steam -client and then connect to your friends in LAN.

https://support.steampowered.com/kb_article.php?ref=3160-agcb-2555

[ReBoot]

Steam has a useful offline mode. Just make sure the "Don't save account credentials on this computer" check box is unchecked. And for best results, make Steam remember your password.

[Coz takhle dat si spenat ?]

Many thanks for update !