NAT-PT would allow IPv4 nodes to communicate with IPv6 nodes transparently by translating the IPv6 address into a registered V4 address. However, NAT-PT would fall flat when the pool of V4 addresses is exhausted. NAPT-PT multiplexes the registered address’ ports and will allow for a maximum of 63K outbound TCP and 63K UDP sessions per IPv4 address, but it is unidirectional. We present in this paper a novel solution ENAT-PT（an enhanced NAT-PT），which will allow for a great number of inbound sessions by using a single V4 address. By using ENAT-PT, we can visit V6 networks from a V4 network with a small address pool.
Zeng, Li-An; Cheng, Zhao-Hui; Ling, Li; Qian, Songrong; and Liao, Xu, "ENAT-PT: An Enhanced NAT-PT Model" (2002). ICEB 2002 Proceedings. 69.