CIDR Notation Explained: What Does /24 Actually Mean

CIDR notation — the “/24” or “/16” that follows an IP address in network configuration — compactly encodes exactly how large a network is, and once the underlying binary math clicks, reading any CIDR block at a glance becomes straightforward.

What the number after the slash actually means

The number after the slash (the prefix length) tells you how many bits, out of the 32 total bits in an IPv4 address, are fixed as the “network” portion — with the remaining bits available for individual host addresses within that network. A /24 means 24 of the 32 bits are fixed for the network, leaving 8 bits (32 − 24) free for host addresses. A /16 means 16 bits are fixed, leaving 16 bits free for hosts — a dramatically larger address range than /24.

From prefix length to address count

The number of free host bits directly determines how many addresses the block contains, following a clean power-of-two relationship: 2^(free bits) total addresses. A /24, with 8 free bits, contains 2⁸ = 256 total addresses. A /16, with 16 free bits, contains 2¹⁶ = 65,536 total addresses — 256 times more than a /24, since each additional free bit doubles the address count.

Why /24 gives 254 usable addresses, not 256

This is the detail that trips up almost everyone learning CIDR for the first time: a /24 block contains 256 total addresses, but only 254 are actually usable for hosts. The first address in any block is reserved as the network address (identifying the network itself, not any individual device on it), and the last address is reserved as the broadcast address (a special address that, when sent to, reaches every device on that network simultaneously). Both are structurally reserved and can’t be assigned to an individual host, which is exactly why usable host count is always total addresses minus 2 — 256 − 2 = 254 for a /24, 65,536 − 2 = 65,534 for a /16.

Reading a CIDR block’s actual address range

Given a network address and prefix length — say, 192.168.1.0/24 — the usable range runs from the first address after the network address (192.168.1.1) to the last address before the broadcast address (192.168.1.254), with 192.168.1.0 itself reserved as the network address and 192.168.1.255 reserved as the broadcast address. A /16 block like 10.0.0.0/16 spans a much larger range — 10.0.0.0 through 10.0.255.255 — with only the very first and very last addresses in that entire range reserved, since the reservation applies once per block regardless of the block’s size.

Why smaller prefix numbers mean bigger networks

This is genuinely counterintuitive the first time you encounter it: a smaller number after the slash describes a larger network, not a smaller one — /16 (65,534 usable hosts) is a much bigger network than /24 (254 usable hosts), even though 16 is a smaller number than 24. The relationship makes sense once you remember what the number actually represents: it’s counting fixed network bits, and fewer fixed bits means more bits are left free for host addresses, which is exactly why a smaller prefix number corresponds to more available addresses, not fewer.

Common CIDR blocks worth recognizing on sight

A few prefix lengths come up often enough to be worth recognizing immediately rather than recalculating each time: /32 is a single address (no free bits at all, used to reference exactly one specific host), /30 gives 4 total addresses (2 usable, common for point-to-point links between two devices), /24 gives 256 total (254 usable, an extremely common size for a small office or home network segment), and /8 gives over 16 million addresses (used for very large allocations, like an entire organization’s internal address space).

CIDR	Total addresses	Usable hosts
/32	1	1 (single host)
/30	4	2
/24	256	254
/16	65,536	65,534

Working it out directly

The CIDR Calculator on this site takes any CIDR block and returns its full usable address range, network address, and broadcast address directly, removing the need to work through the binary math by hand. For deeper subnetting scenarios — splitting a larger block into multiple smaller subnets — the IP Subnet Calculator and IPv4 Subnet Calculator on this site cover that directly, and the IP Subnetting Guide covers the fuller subnetting picture beyond a single block’s basic address count.