1. A computer-implemented method of providing multi-level security systems, comprising: defining, for each of a plurality of security classifications to be used by a multi-level security (“MLS”) system when sending outbound packets to at least one communication partner system, a unique source address; and for each of the outbound packets sent from the MLS system, using the unique source address defined for a particular one of the security classifications which is associated with the outbound packet as a source address in the outbound packet, thereby conveying the security classification of each of the outbound packets through the source address used in the outbound packet, further comprising: determining the security classification associated with the outbound packet; determining the unique source address defined for the determined security classification; replacing a source address field of a packet header of the outbound packet with the determined unique source address; and forwarding the outbound packet for transmission to one of the at least one communication partner system.