CCIE Practical Studies, Volume I

Chapter 15. Configuring Network Address Translation (NAT)

The phenomenal growth of the Internet spawned a tremendous squeeze on IP address space. Some of the stronger solutions offered to relieve this pressure are classless interdomain routing (CIDR) and IPv6. CIDR can be viewed as the short- term solution until IPv6 becomes the dominant version of IP in use. But many private networks and ISPs have yet to migrate to IPv6. Perhaps one reason for the delay to migrate to IPv6 is the huge success of an intermediate solution called Network Address Translation (NAT).

NAT allows many private companies and individuals to implement private address space for their networks, provided under RFC 1918, thereby conserving valuable public address space. NAT also provides ways for addresses in the same routing domain to "overlap" each other yet reach the common hosts . The translation of addresses by NAT inherently provides a level of security directly needed when transporting data across the Internet. This chapter discusses how NAT operates and covers NAT terminology. It also explores the three major implementations of NAT ”NAT pools, static NAT, and NAT overload.