Network address translator and secure transfer device for interfacing networks

An interface for a network for interfacing flows of packets between Internet Protocol ‘IP’ addressable devices in the network and IP addressable devices external to the network, each packet in each flow of packets containing respective fields for a source IP address, a destination IP address, a source port and a destination port, the interface having:

• an address translator coupled to the IP addressable devices for translating addresses of the packets flowing to and from the IP addressable devices,
• a secure transfer device coupled to the address translator for maintaining secure paths for each of the flows of packets to and from the IP addressable external devices, each of said secure paths having a path identifier, the secure transfer device being arranged for each secure path to insert the path identifier for said secure path into the source port field of each packet flowing on said secure path into the secure transfer device, and
• the address translator being arranged to store the path identifiers inserted into said source port fields of said packets flowing into the secure transfer device in order to maintain a record of which of the secure paths corresponds to which of the packet flows.