i have noticed certain sites which allows limited hit per IP so can i programatically make them feel that requests are not coming from the same IP ,
well i am not
No, spoofing your IP is not something you can do in your HTTP headers.
Firstly, I'd suggest that whatever limit you're trying to get around - don't bother. It's there for a reason, and you'd probably be breaking someones terms of use for a service.
Secondly, if you're absolutely determined, I'd say the only way you'd be able to make it look like the request was coming from a different IP would be to actually make it come from a different IP - ie, by using a bunch of proxies.