I have opened ports 6112-6119 on TCP AND UDP on both my router and Firewall and still getting the infamous error that i cannot create or join games on bnet with diablo 1 but i can chat. Is there anything else i can try for this to make it work.
Plug in your internet cable to the wall jack not the router, and it dissapears and you can host games and everything!
Skipping the router is not only often not possible (wifi, fibernet, coax, mobile), but also a security risk.
If you do not have the option to forward the ports in your router then you could play multiplayer using DevilutionX as the client.