Hello Student .
DNB is the diplomat of national board exam which is similar to MD/Ms exam taken by the National Board of Examination that is under the supervision of Health and Family welfare Government of India for Doctor of medicine and Master of Surgeon course of study under the NBE of Health and Family Welfare, Government of India. Diplomate of National Board a postgraduate degree Master's degree awarded by Diplomate of National board to the specialist doctors after their 4years of residency.
Hope this helps.