Each time an ESXi host is added to the vCenter Server, the vCenter Server's IP address is recorded on the ESXi host. If the 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 the vCenter Server after about a minute.
For example, the vCenter Server system might be on the main network (10.10.10.0), and some ESXi hosts might be behind a firewall in the 192.168.x.x network. If the hosts cannot reach the main network, they will be disconnected from vCenter Server.
VMware vCenter Server
VMware vSphere ESXi