NAT and IP Addressing

Network Address Translation (NAT) is a method for rewriting the source and/or destination addresses of IP packets as they pass through a NAT device, which is often a router or firewall that separates two realms or domains on the Internet. NAT was first officially proposed (RFC1631) in 1994 as a temporary solution to the problems of IP address space depletion and the rapidly increasing size of route tables. Addresses, at that time, were divided into two classes: local and global addresses. Today we normally refer to these addresses as either private or public, and the private IP space often is referred to as RFC1918 addresses. Per RFC1918, the Internet Assigned Numbers Authority (IANA) reserved three blocks of the IP address space for private internets:

• 10.0.0.0–10.255.255.255 (10/8 prefix)
• 172.16.0.0–172.31.255.255 (172.16/12 prefix)
• 192.168.0.0–192.168.255.255 (192.168/16 prefix)

NAT commonly is used to enable multiple hosts on private networks to access the Internet using a single public (Internet routable) IP address. Note that although NAT most commonly is used to map IP addresses from internal private IP space to the public IP space, NAT can be used to map between any two IP address domains. Additionally, NAT provides a security function by segregating (hiding) private hosts from the publicly routed Internet. This short-term kludge has had an enormous impact on the day-to-day functioning of the Internet, and has special relevance to system administrators who are charged with securely transporting VoIP packet data across network boundaries.