NAT = Route

Hi All,
I am new asterisk, i want to know more about NAT=ROUTE option for one extentions…

And i want to know this is possible
UA1 <–> Asterisk <–> UA2

UA2 is giving port 6000 in its answer, but its sending RTP using port 7000 (port 6000 is not opened for listening also). Asterisk is getting RTP from 7000, so Asterisk send RTP packets to 7000 ?

I want Asterisk to do that ? is that possible ?

If any one know the answer please give me a solution…

Thanks in Advance
Karthikeyan R