Unable to ping host only adapter

[gmc]

I am running VB on Windows 7 and have a Linux guest.

The VirtualBox Host-Only Network adapter is setup on Windows with 172.21.0.1/24
Not using DHCP.

On the VM Manager I've setup attached to host-only adapter and virtualbox host-only ethernet adapter.

Then on Linux I've give the network interface 172.21.0.2/24

From Windows I am unable to ping the linux guest, and likewise from the guest I am unable to ping 172.21.0.1

Any ideas why - should I be able to using the host only adapter?