How to Calculate the Number of IP Addresses in a CIDR Block

This comes up constantly when sizing infrastructure: given a prefix length, how many IP addresses does that actually give you? The formula is simple once you have it, and it applies to any prefix from /0 to /32.

The formula

Total addresses = 2^(32 - prefix length)
Usable hosts = Total addresses - 2   (for most prefixes - see the /31 and /32 notes below)

IPv4 addresses are 32 bits. Subtracting the prefix length from 32 tells you how many bits are left for host addresses, and 2 raised to that power gives you the total address count for the block.

Worked examples

Step-by-step: how many addresses in a /27?

1. Host bits = 32 - 27 = 5
2. Total addresses = 2^5 = 32
3. Usable hosts = 32 - 2 = 30

The exceptions: /31 and /32

The "subtract 2" rule has two documented exceptions:

• /31 has exactly 2 addresses, and per RFC 3021, both are usable - this is a special case designed for point-to-point links where there's no need for a separate network or broadcast address.
• /32 identifies a single specific address with no network or broadcast concept at all - it's used for host routes, not a subnet in the traditional sense.

Sizing a subnet for a known device count

This formula also works in reverse when you know how many devices you need to support. To fit 40 devices: 2^n - 2 must be at least 40. 2^5 - 2 = 30 (too small), 2^6 - 2 = 62 (fits, with room to grow) - so you'd need at least a /26.

Common mistakes

• Forgetting to subtract 2 for the network and broadcast addresses on standard subnets.
• Applying the "subtract 2" rule to a /31, where it doesn't apply.
• Off-by-one errors when counting host bits - always subtract the prefix from 32, not from 30 or some other number.

Summary

2^(32 - prefix) gives you the total address count for any CIDR block, and subtracting 2 gives you usable hosts for anything larger than a /31. It's a small formula that covers subnet sizing for essentially every practical network design decision.