I've used FSNav, faithfully for the last few years. Back when I was doing allot of Multiplayer flying, there was an issue with the multiplayer interface and FSNav. It would work, but, 1,2 or all of us in the session would have problems with the connections. I know the same IP was used with the session and FSNav. The only other IP used was the one for Roger Wilco. But, we still ran into problems.

You might want to check the FAQs at www.fsnavigator.com to get an answer from the source.