Hi Meghna, For branches like Case,mech,ece,civil u need to get at least 1000 to 1500 rank if u are in open category. There are other small branches like metallurgy , marine,civil environmental. For getting a seat in these small branches u need not have a good rank like 1000 to 1500. Ranks from 4000 to 5000(open category ) are enough . With a ranl of 5000 you can get civil environmental branch. Civil and civil environmental don't have much difference.the difference is in 4th year u subjects will differ from civil subjects. 

Hope this helps, All the best !