How many usable hosts does a /27 give, and why do you subtract 2?

A /27 leaves 5 host bits, so 2^5 = 32 addresses but only 2^5 − 2 = 30 are usable.

Working:

• /27 → host bits = 32 − 27 = 5
• Total addresses in the block = 2^5 = 32
• Usable hosts = 2^5 − 2 = 30

Why subtract 2?

• The all-0s host pattern is the network address (identifies the subnet) — not assignable.
• The all-1s host pattern is the broadcast address — not assignable.

So every subnet loses exactly two addresses to these reserved values, regardless of size. (A /30, the smallest practical subnet, gives just 2^2 − 2 = 2 usable hosts — perfect for a point-to-point link.)

Go deeper:

• RFC 3021 — Using 31-Bit Prefixes on IPv4 Point-to-Point Links (IETF) — the exception where a /31 gives 2 usable hosts with no broadcast.
• ipcalc (jodies.de) — enter any /27 and see exactly which two addresses are reserved.