I have a default route 0.0.0.0/0 pointing to my backhaul provider.
I also have my own /20 IP assignment.
I have clients who do not have public addresses are are Natted at our edge router.
These clients when they do a IP Whois test are showing up as the IP of my provider.
I want these clients to be natted but to use an IP from our own assignment and not my providers so when they do an IP Whois lookup, it will show our own IP address.
Is this possible and how.
Sorry, but may be I read it wrongly at first your question. You mean that you want to assign of the IP that the ISP has given you, to the devices of your LAN?
If that is the case you should do netmap. http://wiki.mikrotik.com/wiki/Manual:IP/Firewall/NAT#1:1_mapping