Subnetting is arithmetic, not intuition. Once you know the method, you can stop guessing and answer the question in under a minute.
The only numbers you need
An IPv4 address is 32 bits. The prefix length counts the network bits.
/24leaves 8 host bits: 256 addresses, 254 usable./25leaves 7: 128 addresses, 126 usable./26leaves 6: 64 addresses, 62 usable./27leaves 5: 32 addresses, 30 usable./28leaves 4: 16 addresses, 14 usable.
Host count = 2^(32 - prefix) - 2, except /31 and /32 which are special.
Finding the network
For 192.168.10.130/26:
- The block size is 64.
- 130 falls in the 128–191 block.
- Network:
192.168.10.128. - Broadcast:
192.168.10.191. - Usable:
192.168.10.129to192.168.10.190.
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