PING having latency when i power on VM?

Hi All,

I have VMware vSphere 4.1 server with two virtual machines. when i ping the ESXi ip pinging normally when one VM running.when i power on another VM ping have lot of drops and i checked out MAC address for those VMs, MAC are different from one to other. what is the issue? how can i rectify that? Both VMs connected to same single vNIC and same port group. No VLANs configured.