Network Address Translation (NAT) is a function by which IP addresses within a packet are replaced with different IP addresses. Routers, load balancers, and firewalls most commonly perform the function. This chapter and the next focus on NAT within routers.
The acronym NAT is used interchangeably to mean network address translation (the function itself) and network address translator (software that runs the NAT function).
When the first edition of this book was published, NAT was understood to mean IPv4-to-IPv4 translation. You might see funny acronym variations such as NAPT, but it remained an IPv4 thing. Now, of course, you have IPv6. And with it, you have a new kind of NAT ...