Rewrite url setting in web.config

<system.webServer>
    <rewrite>
      <rules>
        <rule name="Redirect to WWW"  enabled="true" stopProcessing="true">
          <match url=".*" />
          <conditions>
            <add input="{HTTP_HOST}" pattern="^(domainname).(com)$" />
          </conditions>
          <action type="Redirect" url="http://www.{HTTP_HOST}/{R:0}"
               redirectType="Permanent" />
        </rule>
      </rules>
    </rewrite>
</system.webServer>