All public IPv4 addresses that transverse the Internet must be registered with a Regional Internet Registry (RIR). Organizations can lease public addresses from an SP, but only the registered holder of a public Internet address can assign that address to a network device. However, with a theoretical maximum of 4.3 billion addresses, IPv4 address space is severely limited. When Bob Kahn and Vint Cerf first developed the suite of TCP/IP protocols including IPv4 in 1981, they never envisioned what the Internet would become. At the time, the personal computer was mostly a curiosity for hobbyists and the World Wide Web was still more than a decade away.

With the proliferation of personal computing and the advent of the World Wide Web, it soon became obvious that 4.3 billion IPv4 addresses would not be enough. The long term solution was IPv6, but more immediate solutions to address exhaustion were required. For the short term, several solutions were implemented by the IETF including Network Address Translation (NAT) and RFC 1918 private IPv4 addresses. The chapter discusses how NAT, combined with the use of private address space, is used to both conserve and more efficiently use IPv4 addresses to provide networks of all sizes access to the Internet. This chapter covers: