Question 1

What is the quickest way to subnet?

Accepted Answer

Use the 'block size' (a.k.a. magic number) method: block size = 256 − the interesting octet of the mask. For a /26 the mask octet is 192, so 256 − 192 = 64. Subnets then start at multiples of 64 (.0, .64, .128, .192), which instantly gives you network and broadcast boundaries.

Question 2

How do I calculate usable hosts from a CIDR prefix?

Accepted Answer

Count the host bits: 32 − prefix. Usable hosts = 2^(host bits) − 2, subtracting the network and broadcast addresses. A /24 has 8 host bits → 2^8 − 2 = 254. The exception is /31, used for point-to-point links, which has 2 usable addresses by RFC 3021.

Question 3

Why subtract 2 for network and broadcast?

Accepted Answer

The first address in a subnet identifies the network itself and the last is the broadcast address, so neither can be assigned to a host. That's why a /24 gives 254 usable hosts, not 256. Modern /31 point-to-point links are the documented exception.

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