Virtual machine can't ping host on fresh vSphere5.1 install

Hi,

I've done a fresh install to vSphere 5.1 (previously 4.1 installed which works well). No vCenter for the moment.

One virtual machine on LAN is running Paessler PRTG under Win 2k3 x64.

This virtual machine has been moved from 4.1 to 5.1, nothing has changed.

This VM can't ping the Host. No WMI monitoring too.

A physical computer on the LAN can ping the vSphere host without any problem.

What's wrong ?

Thanks,

Vincent