I will give rough estimate, it may slightly vary from year to year.

1.IISc

It is first preference of every topper. Its very difficult to get into IISc, I will just give an example, there are around 15-20 seats in IISc for structures, so to get into structure you should be within 20, if you are lucky it may go upto 25, if some topper wont take it. So to get some branch in IISc you should be within 100 ranks , it may go upto 150-200 if you have some reservation.

2. IITs

you can check online for individual cut offs of each IIT. You can refer this link IIT Gate Score Cutoff for M.Tech Admission

I am just giving rough estimates, based on my experience and friends rank who got into different IITs. Instead of cut of by college, let may say in terms of specializations (Which is not found any where online)