To connect real HP-IL devices you use real cables. For virtual IL devices you need matching IP port numbers. With VilMA it is now as simple as plugging real cables.
What is VilMA? It's a little REXX routine to handle virtual controllers (V41 or Emu71) and virtual HPIL devices, foremost those from Christoph Gießelink.
Edit: When I programmed VilMA more than three years ago, I heard on "grapevine radio" someone else will do somethng similar. To avoid direct influence I did not publish the source of VilMA. But, after such a long time I changed my opinion -- here it is.