CISCO DNS Application Level Gateway
Cisco IOS contains a sophisticated DNS application level gateway. The literature for which (Actually Cisco have a number of papers on this) requires an intimate knowledge of the DNS and NAT to understand (CISCO appear to have released the technical whitepapers as marketing whitepapers).
Basically if you wish to plug a network into the Internet, that has used illegal or private addresses, you can do this with a small public IP address space range, utilising Network Address Translation (or Port Address Translation).
Typically however you must supply an additional external DNS. Whilst for insecure connections such as the Internet have two DNS servers (outside and inside) is considered best practise, in some circumstances this is undesirable.
CISCO's DNS ALG product which is included in IOS, allows the router to dynamically rewrite the DNS packets, so you do not need an extra DNS server. There are some restrictions on the redundancy that can be built-in, and applications that can be used. But for coping with corporate mergers the technology is a great stop-gap before the inevitable renumbering.
The author regards NAT and PAT as a horrid stop-gap (roll on IPv6), but if you must have a stop gap, make it work well.
This article was written by Simon Waters when he was working for Eighth Layer Limited.