Network address translators: effects on security protocols and applications in the TCP/IP stack

One proposed method for mitigating the address shortage problem in IPv4 is to use network address translators (NATs) to allow address reuse. The basic idea is to transparently map a wide set of private network addresses and corresponding TCP/UDP ports to a small set of globally unique public network addresses and ports. NAT devices provide a way to handle IP address depletion incrementally, without changing hosts and routers, until more long-term approaches like IPv6 can be implemented. Existing Internet security protocols must be re-examined, however, to see how they function within this new network environment. We begin with a description of the four NAT environments and a discussion of their limitations. We then examine the relationships between NAT devices and popular Internet security protocols and applications at each layer of the TCP/IP stack to see if they can survive with NAT devices