Hi,

If you want to apply for JEE Mains after diploma then you will be treated just like any other student who has passed 12th. No special benefits are provided to diploma students and they are also given admission in BTech first year.

If you want to take advantage of diploma then you must also apply for LEET exam which is a national exam for diploma holders. Through thus you can get admission in second year of BTech or BE directly. Though diploma holders aren't eligibile for NIT yet they are eligible for IIT.