Diploma holders are eligible to get direct admission in second year of B.Tech or B.E. The exam through you can avail this facility is konwn as LEET EXam

LEET - Lateral Entry Entrance Exam

LEET exam is conducted for Diploma holders who want to get admission in B.E or B.tech courses. This is national level exam. But you can also apply for state level exams in you belong.

In some state there is no exam admission is given on the basis of percentage in diploma. for e.g Maharashtra admission in direct 2nd year is done on the basis of diploma marks.through this exam yiu can get state government engg collage or other institutes

Sorry budy but there is no direct exam to get admission in IIT/NIT after complte a diploma course.