Hyper-V VMs ping host, but not each other... why?

I've got my Hyper-V Server and created 2 VM's (both identical with different computer names, both on the domain, different static IPs, on same subnet, same network mask, etc).

My virtual switch is set at External with the default settings. My server has 1 NIC connected and it's using that so the vm's can hit the internet.

Problem is, the VMs themselves cannot talk to each other but they can both ping and get to resources from my Hyper-V server.  

What do I have to do to fix this? 

My static IP settings are:
172.16.100.20-25 (node ips I'm going to use)
172.16.100.1 (router)
172.16.100.10 (hyper v server ip)
255.255.240.0 (network mask)