Communications relay device, program and method, and network system

The present invention relates to a communications relay device placed between networks. The communications relay device includes a unit of translating a content of a call control packet flowing between the networks, a unit of storing translation information for translating a content of a data packet for each session, a unit of translating a data packet in accordance with the stored translation information, a unit of analyzing a content of a call control packet flowing between a first communication device placed in the first network and a communication device for call control and registering translation information, and a unit of associating two pieces of translation information generated based on call control packets relevant to a session between communication devices placed in the first network.