LAN gaming, bug or feature?

I hosted a gaming server on another PC in a network, to be played on by PC's in the same network.

With v3, every single machine was set to strict protection/no sharing, and this WORKED in the past perfectly fine.

With v4 I get a series of "ARP attacks" and the server is not found, until I enable network sharing in ESET on the machine playing, then I can find the server, and I no longer get logged attacks:



.23 = another player
.24 = server
.21 = me
.1 = router

The server itself did not have a security package installed, only the gaming machines.

So my question, why do I suddenly need network sharing in v4 for this to work? Is it a bug?

Using Vista SP1 (new filtering)