I had an issue where we built 6 Hyper-V Windows 2008 virtual machines with IIS that needed to connect to an SQL database that was on the Hyper-V host system.
We could not make the connection from the websites, nor could we ping or telnet to the Hyper-V host. The Hyper-V guests could ping and connect to any other system on the network, and the Hyper-V host could be connected to by any other system on the network.
The issue ended up being that we had used network teaming on the interfaces that hosted the virtual network for the Hyper-V guest operating systems. Hyper-V does not support network teaming, so we removed the network team, and the software and just used one of the physical network interfaces to create the Hyper-V virtual network.
Connectivity was restored after removing the network teaming configuration and then teaming software, and then rebooting the Hyper-V host server.