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Endless questions on network and broadcast addresses, host ranges, masks and prefixes. Answer, then see the method worked out step by step, until subnetting takes you seconds.
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Subnet mask
What is the dotted-decimal subnet mask for /30?
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Guide
Write the prefix as a mask. The interesting octet is the one that’s neither 255 nor 0: for a /20 (255.255.240.0) it’s the third.
If the prefix is a multiple of 8 (/8, /16, /24), there’s no interesting octet: keep the network octets and zero the rest.
Subtract the mask’s value in that octet from 256. For 240 that’s 16, so networks start at 0, 16, 32, 48 and so on in that octet.
The network starts at the multiple of the block size at or below the address’s value in that octet; every octet after it is 0.
The broadcast is the address just before the next network. Usable hosts are everything in between.
Usable hosts = 2^(32 − prefix) − 2. Subnets made by borrowing bits = 2^(bits borrowed).
Practice until each question takes under 30 seconds; that’s the pace exams like Network+ and CCNA reward.
Network, broadcast, host range, masks and the binary behind them, for any IPv4 address and prefix.
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Aim for under 30 seconds per question. Network+ and CCNA questions often hide a subnetting step inside a bigger scenario, so speed leaves you time for the rest.
Easy keeps to /24–/30 in 192.168.x.x, so only the last octet changes. Medium uses /16–/30 across private ranges. Hard uses any prefix from /8 to /30 on any unicast address.
For prefix questions either works: /26 or 255.255.255.192. Address questions need a dotted-decimal address.
A+ covers IP addressing basics such as private ranges, APIPA and default gateways. Network+ and CCNA test full subnetting.
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