For eg:- you have the following sub-domains associated with your main domain.
www.example.com - Main domain
google.example.com - Sub-domain
Yahoo.example.com - Sub-domain
Bing.example.com - Sub-domain
You are required to get all the above verified one by one via webmaster tools. As you keep on adding sub-domains you have to repeat verification process. HARD ISn't ? :-(
Think your hard times are over and your good times have blossomed. A short cut to this perennial issue has been discovered by Google and it is none other than the simple DNS verification process to prove that you have enough rights to modify the website. By doing so all of your sub-domains associated with your main domain will automatically get verified in Google webmaster tools.
For those who do not know how to add a DNS record can well opt for other verification methods viz. upload a HTML / Add a meta tag.

Screenshot of webmastertools third new verification method "add a DNS record"

Screenshot - DNS txt record creation in a domain DNS configuration panel.








