Authentication is one of the foremost features of web application's security. Authentication is a process of identifying a user. ASP.NET implements authentication using authentication providers, which are code modules that verify credentials and implement other security functionality such as cookie generation. ASP.NET supports the following three authentication providers:

1. Forms Authentication:
Using this provider causes unauthenticated requests to be redirected to a specified HTML form using client side redirection. The user can then supply logon credentials, and post the form back to the server. If the application authenticates the request, ASP.NET issues a cookie that contains the credentials or a key for reacquiring the client identity. Subsequent requests are issued with the cookie in the request headers, which means that subsequent authentications are unnecessary.

2. Passport Authentication:
This is a centralized authentication service provided by Microsoft that offers a single logon facility and membership services for participating sites. ASP.NET, in conjunction with the Microsoft Passport software development kit (SDK), provides similar functionality as Forms Authentication to Passport users.

3. Windows Authentication:
This provider utilizes the authentication capabilities of IIS. After IIS completes its authentication, ASP.NET uses the authenticated identity's token to authorize access.

To enable a specified authentication provider for an ASP.NET application, first we create an entry in the application's configuration web.config file as follows-

<authentication mode = “[Windows/Forms/Passport/None]”>
</authentication>

Combinations between IIS Authentication and ASP.NET providers.