 the subdivision of a class-based network into subnetworks. Today, it generally refers to the subdivision of a CIDR block into smaller CIDR blocks. Sub- netting allows single routing entries to refer either to the larger block or to their individual constituents, which permits a single general routing entry to be used through most of the Internet, with more specific routes required only for routers in the subnetted block. Network Address Translation (NAT) is another technique used to deal with the shortage of IPv4 addresses. NAT was developed specifically to address this problem when the cost of extra IP addresses is an issue. NAT is therefore of particular inter- est in countries other than the United States where historically there have been fewer addresses allocated per capita, and also in small businesses and home offices. NAT enables a LAN to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. The necessary IP address translations are per- formed at the NAT router, which is located where the LAN meets the Internet. NAT allows an enterprise to use internal IP addresses, which, because they are internal, present no conflict with IP addresses employed by other enterprises. In a typical configuration, the LAN uses one of the designated "private" IP address sub- nets (10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16). The router connecting this LAN to the Internet will have an address from the private subnet and a public address assigned by the ISP. The NAT function in the router translates between the public and private addresses. As far as the Internet is concerned, the router is the source and destination for any traffic. Because NAT-enabled routers do not have end-to-end connectivity, they cannot take part in some of the Internet protocols; for example, security protocols such as IPsec are complicated by the use of NAT- although the IETF has approved an extension to IPsec that encapsulates IPsec inside UDP, thus enabling it to traverse NAT devices. On the other han