I have deployed one web service in windows azure as a web role.
I have five client which are going to access this web server, what i need is,
1> Nobody can access this web service except this five client.
2> Mechanism through which i can implement authentication by virtue of which client can use service only from his machine, not from other machine some type of domain based authentication or certificate base?
You can go with Forms based authentication. But the right way to do is through WS-Federation for more flexibility. You can go with Forms based +ACS for this OR you can implement WIF to generate Claims and use it as authentication mechanism. Following links
would help you a lot in this.
You can add for each user own certificate. Take a look at ACS management portal section service identities as I wrote :-) Click add and there you select type of identity authentication. Username/password, symmetric key or certificate ;-)Windows Azure Teamleader Cloudikka blog