Many times this is done a little non standard so you would have to research how the ISP recommends you do this. Most what you do is define a nat that maps one of the real ip you have to the DHCP assigned address. The end device does not actually get the ip address. Not sure how BT does it I know ATT 2wire systems also have some strange DMZ option that really does give the IP to the end device but it is dependent on a special feature in the router to do it.
Thanks for your advise, just checked with ISP and requested them to set the NAT PAT on the router (as they actually mange the router, and I do not have login for it) but to my surprise they refused saying as it is leased line router that is how it should be and they will not make any changes to it, rather we should make some provision on our end,
Now is there any other way to set the NAT-PAT on the DHCP server rather doing it on router? Or the ISP just fooling us?
If they are running DHCP on their router and giving you 192.168.x.x addresses then they HAVE to do the nat since only they have the IP. What they may be doing is assigning the LAN port on their router to a actual subnet. This means you need to either hook 5 actual devices behind it or you hook a router behind it that can in effect trick them into sending all the IP to same router. This is done with proxy arp and is pretty easy but you need a actual route and not consumer "gateway" things called routers that can only have a single wan ip.
If they give you 192.168 addresses then I have no clue, I would try to find another ISP. They HAVE to do the nat if they do not pass you the real ips