Why CNAME records are unique in a domain. We can add mail.india.com cname abc.india.com but not again mail.india.com A 10.10.0.1 or mail.india.com cname xyz.india.com If anybody can provide me any link to good document it will do

Hello, it is like that by design. If you want that it also points to another IP address then create another A record named abc with the wanted IP address. You can also create directly A records named mail with the wanted IP addresses without using CNAME records. This posting is provided "AS IS" with no warranties or guarantees , and confers no rights. Microsoft Student Partner 2010 / 2011 Microsoft Certified Professional Microsoft Certified Systems Administrator: Security Microsoft Certified Systems Engineer: Security Microsoft Certified Technology Specialist: Windows Server 2008 Active Directory, Configuration Microsoft Certified Technology Specialist: Windows Server 2008 Network Infrastructure, Configuration Microsoft Certified Technology Specialist: Windows Server 2008 Applications Infrastructure, Configuration Microsoft Certified Technology Specialist: Windows 7, Configuring Microsoft Certified IT Professional: Enterprise Administrator Microsoft Certified IT Professional: Server Administrato

Use CNAME only if you have no other way, how to solve the problem. Short info on cname is here http://en.wikipedia.org/wiki/CNAME_record (more information is in RFC). NS and MX records should NOT point to Cname.