Kerberos Authentication setup in NAV

Hello All,
Right now my NAV setup is working fine with NTLM authentication settings but now I want to switch to Kerberos authentication.

I read in some of NAV forum that Kerberos is the default authentication but If I turn off NTLM setting I am getting unauthorized error on SOAP UI.

Error in detail:

HTTP/1.1 401 Unauthorized
Content-Length: 0
Server: Microsoft-HTTPAPI/2.0
WWW-Authenticate: Negotiate
Date: Mon, 23 Feb 2015 09:30:25 GMT

I have changed the provider in IIS to Kerberos and try again but getting same above error.

Please let me know what I have missed or how can I do this setup?

Thanks in advance!!

-Pooja Kothari