1. A computer program product for providing multi-level security systems, the computer program product embodied on one or more computer-usable storage media and comprising computer-usable program code for: 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 processing each of the outbound packets sent from the first MLS system to the second MLS system, further comprising: 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; and replacing, by 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, the outbound packet to the second MLS system.