The present invention is directed to a secure gateway/router system and method for allowing a MLS network which simultaneously contains data of multiple security levels to interface with one of several MSLS networks which is configured to maintain data with a single security level. When the data is within the secure gateway/router system, the separation of the data by security levels, as it was received from the MLS network, is maintained by routing the data to a designated security level network. Secure gateway/router system includes several network interface units. Each network interface unit is configured to interface within a single MSLS network. Each MSLS software partition has a single and simple function to perform in maintaining separation of security levels and provide the data to the network interface unit configured to interface within the designated security level network.