8. NAT—Network Address Translation
Network Address Translation is a technology to substitute one source or destination address in the IP header with another address. Traditionally, it’s an IP address translation technology to map packets between two different addressing realms. NAT’s most common use is to map outgoing connections between a privately addressed local network and the publicly addressable Internet. In fact, that was what it was originally proposed to do, primarily in conjunction with the then newly defined private class address spaces; both were attempts to alleviate the IPv4 address space shortage.
This chapter introduces the concept of NAT and tells what the various types of NAT are typically used for.