i didn get the point yet that what is diffrence between subnet number and major network number .
In INE practice exams, the major network is your /16 block of addresses. The subnet number tends to be a /24 network, though it can be anything smaller than the overarching /16. So if your major network is 139.X.0.0/16 on rack 13 (so X = 13), and your subnet is .5.0/24, then your network is 126.96.36.199/24.
That make sense?
you mean for example if we have 172.17.0.0 /16 and 172.17.5.0 /24 first one is major and second one is subnet
i mean major always is /16 and any smaller mask can be subnet ?
Yes, this is the case for INE exams. I'm not sure how often the term 'major network' is used in production, especially since most enterprises use private address space, but the idea here is that they own some large block of addresses and subnet it appropriately. It doesn't have to be a /16, but that's the typical case for practice exams.