What is NAPT (Network Address Port Translation)?
NAPT, short for "Network Address Port Translation," is a technique used to translate IP addresses and port numbers contained in TCP (or UDP)/IP packets into a different IP address and port number. It is also referred to as IP masquerade and can be considered an evolution of NAT (Network Address Translation), an IP address translation technology.
NAT involves translating IP addresses in the header of IP packets from one address to another. It is commonly used to allow internal devices assigned private IP addresses (introduced to address the IPv4 address exhaustion issue) to communicate with external servers over the internet using a shared global IP address. However, NAT only supports one-to-one address mapping, which prevents handling simultaneous communication from multiple devices. NAPT was developed to address this limitation.
With NAPT, when it receives packets from devices assigned private IP addresses, it translates the source IP address into a global IP address and modifies the source port number. It records the correspondence between the original and translated addresses, then forwards the packets to the internet. Responses from the server are sent to the modified IP address and port number, which NAPT uses to map back to the original private IP address and port number before delivering the packets to the originating device. The change in port numbers allows NAPT to distinguish between different private IP addresses. As a result, multiple devices can use the same global IP address for simultaneous communication while being uniquely identified by their respective port numbers.
Today, the term "NAT" is often used generically to include NAPT as well. While NAPT inherently provides some level of network concealment by hiding private networks from external entities, this is not a fully reliable security solution. Additional tunneling and end-to-end technologies are required to address comprehensive security threats effectively.