Abstract: Some efficient solutions for making H.323 call through network address translation(NAT) gateway are proposed in this paper. The H.323 protocol packets are unmodified from the source to the destination in endto end network. However, the port and IP address of these packets will be modified when they pass through the NAT gateway. The modified packets address cannot match the calling address in the H.323 protocol packets. This will result the H.323 call abnormal. We present some efficient methods to solve this problem by extending the ALG (application layer gateway) model.

Key words: network address translation, H.323, VoIP