Counting Hosts in a Subnet

The number of addresses in a subnet depends entirely on the host bits — the 0 bits in the mask. The more host bits, the more addresses.

If you know the prefix, you know the host bits: subtract the prefix from 32. A /24 has 32 minus 24 = 8 host bits.

The total number of addresses in a subnet is 2 to the power of the host bits. With 8 host bits, that is 2^8 = 256 total addresses.

This count includes every possible combination of host bits, from all zeros to all ones — including the special network and broadcast addresses.

Two addresses in every subnet are reserved: the network address (all host bits 0) and the broadcast address (all host bits 1). Neither can be assigned to a device.

So usable hosts = total minus 2. A /24 gives 256 - 2 = 254 usable hosts, the figure you will quote most often.

Combine the steps into one rule: usable hosts = 2^(32 - prefix) - 2.

• /24 → 2^8 - 2 = 254
• /25 → 2^7 - 2 = 126
• /26 → 2^6 - 2 = 62

Each step up in prefix roughly halves the usable hosts, since you remove one host bit.

Tiny subnets are common for router links:

• /30: 2^2 - 2 = 2 usable hosts — perfect for a two-router point-to-point link.
• /29: 2^3 - 2 = 6 usable hosts.
• /28: 2^4 - 2 = 14 usable hosts.

These efficient blocks waste almost no addresses.

Smaller prefixes hold many hosts:

• /16: 2^16 - 2 = 65,534 usable hosts.
• /22: 2^10 - 2 = 1022 usable hosts.
• /20: 2^12 - 2 = 4094 usable hosts.

Choosing the right size avoids both running out of addresses and wasting them.

A /31 normally would give 2^1 - 2 = 0 usable hosts, which seems useless. A special rule (RFC 3021) lets a /31 be used on point-to-point links with exactly 2 usable addresses and no broadcast.

The exam may mention this; just remember a /31 is a modern, efficient choice for router-to-router links.

A /32 has zero host bits, representing a single specific address. It is used in firewall rules, loopback interfaces, and host routes.

You do not count usable hosts for a /32 in the usual way — it simply identifies one exact address rather than a range of devices.

To pick a prefix, find the smallest subnet that still fits your hosts plus the two reserved addresses. Need 50 hosts? A /26 (62 usable) fits; a /27 (30) does not.

Always round up to the next size and leave room for growth, but avoid grabbing far more than you need and wasting address space.

For fast mental math, memorize key powers of two: 2^6=64, 2^7=128, 2^8=256. Then subtract 2.

If asked how many hosts fit a /27, recall 2^5 = 32, minus 2 = 30. Building this reflex makes subnetting questions quick to answer under exam time pressure.

Counting hosts tells you whether a subnet meets a requirement and how many addresses remain free. It is the metric that drives every subnet-sizing decision.

Next you will use these counts to actually carve one network into several subnets, completing the core subnetting skill.

Calculate usable hosts.

You learned to count subnet hosts.

• Host bits = 32 - prefix.
• Total addresses = 2^(host bits).
• Usable hosts = 2^(32 - prefix) - 2 (minus network and broadcast).
• /24 = 254, /26 = 62, /30 = 2 usable hosts.
• Special cases: /31 (2 hosts, point-to-point) and /32 (single address).