Stateless NAT router

Hello!

Is there any quality ethernet-to-ethernet routers around which will do
stateless NAT? That is, the router should have set of internal address and a
set of external address. The router is then given rules of the form:

Internal address 10.1.2.3 TCP port 42 should be mapped with external
address 294.58.4.3 port 75

We have a cisco router with IOS which allow this kind of rules. However, the
router seems to keep track of the connections made and we are worried about
this since we want to be able to support _very_ many connections (hundreds
of thousands), both udp and tcp.

Best,

Hans