1.Because you are using intranet application you first option should use Windows Authentication.
To enable Windows Authentication within an ASP.NET Application, you should make sure that you have “Integrated Windows Authentication” enabled within IIS for the application you are building.
Then you should add/modify the next settings in your web.config file:
<configuration>
<system.web>
<authentication mode="Windows" />
<authorization>
<deny users="?"/>
</authorization>
</system.web>
</configuration>
2.Second solution could be to implement Forms authentication, like for an internet application.
In this case you can start from the next:
http://msdn.microsoft.com/en-us/library/vstudio/7t6b43z4(v=vs.100).aspx[
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