Thanks for your reply.

Yes, VM A and VM B are VM running on Workstation with operation system ESXi.

I capture packets in NAT network of workstation using wireshark. And MAC 00:xxxxxx:b2 is the new MAC generated by workstation automatically. But in wireshark, the packet comes from 00:xxxxx:b2 has MAC 00:xxx:4C. MAC 00:xx:4C should belongs to VM A! 

And this weird network works well except VM A and VM B can't connect to each other.

You can reproduce this by following steps:

1. create a ESXi VM on vmware workstation.

2. clone this VM.

3. boot new VM, check this VM network card's MAC.

4. restart new VM's network manager to let it do DHCP operation, and use wireshark to capture DHCP packets.

you will see MAC is weird.