Hi Ramesh,
Where did you configured the public IP whcih given by your ISP?
If you are full with interfaces then try to configure the public IP as a secondary IP on the interface to which your ISP link connected.
Once you configured the poublic IP on interface then you can configure NAT as well PAT.
Let me know if you need any configuration help.
Another question is why you used to interfaces for LAN only? what is the purpose? what you are looking to achieve?
Please rate the helpfull posts.
Regards,
Naidu.