1. A computer-implemented method of providing multi-level security systems, comprising steps of: defining, for each of a plurality of different security classifications to be used by a first multi-level security (“MLS”) system when sending outbound packets to a second MLS system, a unique source address; and for each of the outbound packets sent from the first MLS system to the second MLS system, performing steps of: determining, by the first MLS system, a particular one of the security classifications that applies to the outbound packet; locating, by the first MLS system, the unique source address defined for the determined particular one; replacing, the first MLS system, a source address in a packet header of the outbound packet with the located unique source address, thereby conveying the security classification that applies to the outbound packet through the source address in the packet header; and sending, by the first MLS system responsive to the replacing step, the outbound packet to the second MLS system.