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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  IP Network Address Translation (NAT) Protocol

IP NAT Address Terminology 1 2 IP NAT Unidirectional (Traditional/Outbound) Operation

Comparing Static and Dynamic Mappings

The trade-offs between static and dynamic NAT mappings are pretty much the same as they always are when the choice is between “static” and “dynamic”; for example, the same issues arises in ARP caching. Static mappings are permanent and therefore ideal for devices that need to be always represented with the same public address on the outside network. They may also be used to allow inbound traffic to a particular device; that is, for transactions initiated on the public network that send to a special server on the inside network. However, they require manual setup and maintenance, and they don't allow IP sharing on the internal network.

Dynamic mapping is normally used for regular clients in order to facilitate public IP address sharing, a prime goal of most NAT implementations. It is more complicated than static mapping, but once set up is automatic.

It is possible to mix dynamic and static mapping on the same system, of course. We can designate certain devices that are statically mapped and let the rest use dynamic mapping. We just have to make sure that the static mappings don't overlap with the pool used for dynamic assignment.

Incidentally, another way that dynamic mapping of global and local addressing is performed is through domain name resolution using DNS. This is particularly common when external devices access internal hosts using bidirectional NAT (inbound transactions). Since hosts on the public Internet know nothing about the organization's private network, they issue a request for the DNS name of the device they want to access. This causes a NAT translation entry to be generated that maps the inside local public address of the host to an inside global address for use by those outside the network. See the description of bidirectional NAT for more details on how this works.

IP NAT Address Terminology 1 2 IP NAT Unidirectional (Traditional/Outbound) Operation