Synchronizing two separate networks
Each time server can only synchronize one network. If two or more separate/isolated networks (Such as a 10.10.200.xxx network and a 192.168.000.xxx network) need to be synchronized, either a gateway needs to be installed between the networks to make a subnet or a separate time server needs to be installed on each network. A gateway/router allows NTP traffic to pass but eliminates network isolation. A time server can't sync more than one network without a gateway since a time server's IP address has to match the IP scheme of the network that it is on. The IP address will only allow communication with other network devices with similar IP addresses. If the same address is used on a different network, the network would not be able to get the NTP response from it. For example, a time server IP address of 10.10.200.1 can't communicate with a 192.168.0.xxx network. A gateway enables two or more networks (subnets) to be able to communicate with different IP address configurations.
If network isolation between two or more networks is required, a gateway between the networks is not a viable solution. In this case, use an ethernet time server such as the model 9x88 on each network. Multiple ethernet time servers can be used (for each isolated network) with the same master clock remote output port by daisy-chaining the RS-485 inputs together. Each model 9x88 is then given a network IP address for its network to provide time synchronization to only that network. This method of synchronization keeps all of the networks isolated from each other - a key element of network security.