NAT translates the IP address in the IP datagram header of a packet into another IP address and translates the port number to make the IP address reusable. As a transition technology that alleviates public IPv4 address exhaustion, NAT is easy to implement and therefore widely used.
What Is NAT Used for?
As the number of network applications increases, IPv4 address exhaustion has become a bottleneck hindering network development. Before IPv6 can be widely used to replace IPv4 that has been running on network devices and bearing existing applications, some IPv4-to-IPv6 transition technologies (such as CIDR and private IP addresses) can be used to alleviate IPv4 address shortage. NAT is such a kind of transition technologies.
When a packet sent by an intranet user for accessing the Internet reaches a NAT-enabled gateway, the gateway translates the IP address in the IP datagram header of the packet to another IP address and the port number to another port number, and then forwards the packet to the Internet. In this process, the gateway can translate the IP addresses of packets from different intranet users to the same public IP address and differentiate intranet users based on port numbers. In this way, the public IP address can be reused.
In early times, NAT refers to basic NAT, which is easy to implement and can translate only IP addresses but not port numbers. Therefore, basic NAT can only allow intranet users to access the Internet, but cannot address the IPv4 address shortage issue. Later, Network Address Port Translation (NAPT) came into being, which can translate both IP addresses and port numbers and allows multiple private IP addresses to be mapped to the same public IP address. As such, NAPT alleviates IP address shortage.
NAT Types
Based on whether source or destination IP addresses of packets are translated, NAT is classified into source NAT, destination NAT, and bidirectional NAT.
Source NAT
Source NAT translates only source IP addresses of packets and applies to the scenario where intranet users access the Internet. When a packet sent from an intranet user for accessing the Internet reaches a NAT device, the NAT device translates the private IPv4 address of the packet into a public IPv4 address. In this way, the intranet user can access the Internet.
Based on whether port number translation is performed during source IP address translation, source NAT is categorized into the types shown in the following figure.
