I recall reading about TCP/IP-powered Internet multiplayer DOS Quake in TECHINFO.TXT that shipped with the retail version of the game, and I quote:

  Beame & Whiteside TCP/IP
  ------------------------
  
  This is the only DOS TCP/IP stack supported in the test release.
  It is not shareware...it's what we use on our network (in case you
  were wondering why this particular stack).  This has been "tested"
  extensively over ethernet and you should encounter no problems
  with it.  Their SLIP and PPP have not been tested.  When connecting
  to a server using TCP/IP (UDP actually), you specifiy it's "dot notation"
  address (like 123.45.67.89).  You only need to specify the unique portion
  of the adress.  For example, if your IP address is 123.45.12.34
  and the server's is 123.45.56.78, you could use "connect 56.78".

I looked around a little and sure enough, a copy of the software was avaiable in a subdirectory of idgames(2?) at ftp.cdrom.com. I knew nothing about TCP/IP networking at the time, so it was a struggle to get it all working, and in the end, the latency and overall performance was miserable and totally not worth it. Playing NetQuake with WinQuake was a much more appropriate scenario.