Civilization isn't multiplayer. Only the CivNET, a windows remake of the original is.
Doom can be played over LAN, but don't ask me how. I've seen it being played on LAN.
For
Duke I don't know, but check out this link:
http://dukenews.fragland.net/portal/...hp3?category=5
It has two GREAT ports for DN3D (JonoF and Duke3d W32 (rancidmeat)), for windows that is. And several utilities to play multiplayer over lan or internet with those ports. You need an orginal copy of Duke (1.3d, 1.4 or 1.5) but if both pc's have shareware versions it'll work too.
Dos-only multiplayer, I know nothing about. I have a DOS and WinXP machine and they don't network together...