If I'm not mistaken, and I might be, Heretic [size=9px](like Doom 1 & 2)[/size] was a Pre-Win95 era game. Network gaming was quite primitive back then and usually required the use of accessing hardware ports in some cases.

If you are running almost any modern Windows these days though the game might still run, the port access rules have changed big-time. This might be a part of your problem and would likely require a total rewrite.

It's been ages since I've played these DOS games so I can't remember exactly what the multiplayer networking options were back then. [size=9px](Could you please list them for us for edification purposes?)[/size] But I don't think that TCP or even UDP [size=9px](when was it that games started using this one instead of TCP?)[/size] was available back then.

IPX was popular for some time in the pre-Windows 2000(NT takeover years) OS versions, but I guess it fell into a redundant category since game developers found TCP/UDP to be easier to work with alone I guess. Though I'm sure that you can still install the protocol if you managed to find a friend that would want to come by with his laptop and hook into your LAN.