The_Dev_Crypt

Each time you add an ESX host to vCenter Server, the IP address of vCenter Server is recorded on the ESX host. If the ESX host is behind a NAT and the vCenter Server is on a different network (and thus the address is not accessible due to the NAT), the host disconnects from vCenter Server after about a minute. 

For example, the vCenter Server system might be on the main network (10.10.10.0), and a few of the ESX hosts might be behind a firewall in the 192.168.5.x network. The hosts cannot reach the main network and thus become disconnected from vCenter Server. Continue reading Using NAT between the vCenter Server system and ESX hosts