Mac® Security Bible

15.2. Understanding NAT, DHCP, and IPv6

The current version of the Internet Protocol (IP), version 4, has been in use since 1981. Among other things, IPv4 specifies how devices (or hosts) on the Internet are addressed and how data passes between them. But there's a problem, which is that the format used for creating IPv4 addresses — a dotted quad such as 123.45.67.89, with each segment having possible values from 0 to 255 — can produce, at most, about 4.3 billion unique addresses. That may sound like a lot, but every host on the Internet needs its own address — not only computers but also devices such as mobile phones, printers, DVRs, and even some light switches. When you add up all those devices, that pool of addresses starts shrinking in a hurry. Various estimates put the date of IPv4 address exhaustion somewhere between 2009 and 2011.

Fortunately, a solution to this problem has already been invented and (partially) deployed: IPv6, which uses a new addressing format with more than 340 undecillion possible addresses (that's 340 followed by 36 zeroes). But even though current versions of Mac OS X, Windows, and Linux support IPv6, a lot of the routers and other Internet infrastructure haven't yet been upgraded to work natively with IPv6, and the same goes for quite a few common Internet programs. As a result, an interim solution was needed.

15.2.1. NAT

In a few years, IPv6 will inevitably reach the level of mainstream usage that IPv4 has today. In the meantime, most of us have ...