Hello Sharmila!

Admission into NIT and IIT can be done only via JOSAA. There are no othermeans to get admission into these institutions.

For admission into NIT, candidates sit for the JEE Mains and based on the rank secured by them, they get respective courses in the 31 different NITs across India.

For admission into IIT, candidates sit for the JEE Advanced. But in order to be eligible for JEE Advnced, candidates have to get the required qualifying marks in JEE Mains. Based on the rank secured by the candidate in JEE Advanced, they get the respective streams  in the 23 different IITs across India.