LDAP path not working by IP but working by Domain Name.

Question

I am just trying to athenticate Active Directory Logins . When I give DirectoryPath with domain name eg:- LDAP://Server01.COM. Its Working fine.But with Ip Address eg:- LDAP://192.168.0.68 then asp.net throws an exception   "Server is not Operational" .
Is domain name required for LDAP ?
Is this possible to validate AD logins from workgroup machine?

Please help me.
any help would be appreciated . 

Thanks and regards
Amar