I have 2 pc's with Windows Server 2003 R2 installed and both have the same 3rd party application running. The pc's are connected to two LAN networks (A & B) and the application should only share data between the two pc's over one LAN network (call it LAN A). It was discovered that when physically disconnecting the pc's from the LAN A it was still possible to ping the pc's with their LAN A ip addresses. In other words, somehow a ping over the LAN B was possible with LAN A ip addresses. The ip addresses assigned to the network cards were checked and found to fine, as well as host files for both pc's were checked and found to be correctly configured and identical. However, when the host file from pc 1 was copied to pc 2, this behavior was no longer observed. In other words, the expected result of not being able to ping with LAN A ip addresses over LAN A when the pc's were only connected via LAN B was observed. I should add that there was a DHCP Server running on both machines but this had been disabled and still the same behaviour was noticed. Does someone perhaps know the possible reason for this? Or atleast someone has seen some like this before?

It sounds like you have a few variables in your description. If you PING via IP address, the hosts file is not a factor. The ability to PING another by IP all depends on the way your network is configured. Computers use the local routing table (open a command prompt and type ROUTE PRINT) to determine where to send packets to. Routers do the same. They use their routing tables to determine where to send the packet to next. So its hard to say without knowing your network layout how the packet arrived (path taken) to the LAN A interface. Here is a summary of this process in case you are interested. How IP Packets are Routed on a Local Area Network http://www.anitkb.com/2010/06/how-ip-packets-are-routed-on-local-area.html Once you figure out the routing portion of this scenario, then you can look at other factors such as name resolution (DNS & the HOSTS file). Visit: anITKB.com, an IT Knowledge Base.