CCSP Self-Study: Securing Cisco IOS Networks (SECUR)

Using NAT with IPSec

NAT poses a special problem for IPSec. NAT was developed to forestall the move to IPv6 as IPv4 IP addresses were on the verge of being completely assigned. A NAT border device, such as a firewall or router, translates an internal network IP address into a public network IP address. This device saves addresses because businesses can now assign their own IP address series to internal hosts and permit only those that require a public network to reach the Internet through NAT.

NAT provides a one-to-one address translation. A single internal address gets translated into a single public address. With the rise in the use of web browsers and the availability of information on the web, NAT conversion quickly presented the problem ...