I understand that this game requires a game code to play online. What about LAN, though? Is it possible to play a LAN multiplayer game on several computers with just one purchase? Or would a code for each installed copy be required even for LAN? thanks.

I don't think so.
I'm a new guy here and I bought Defcon.
I play this game some year ago and I tryed to create a lan srv trought hamachi or tunngle... but nothing works... there is a defcon srv creator called "deadcon" but it's useless if you don't own an original copy.
However... on youtube you can check how to play it trought a program called Tunngle.
Best whises!

Haappyy aramgeddon day, dude!