🌐 Subnetting Explained: What CIDR Notation Actually Means

"/24" next to an IP address looks cryptic until you know what it's counting — once it clicks, subnetting stops being a memorization exercise and becomes simple arithmetic.

What CIDR Notation Actually Counts

The number after the slash (e.g., /24) is the number of bits reserved for the network portion of the address, out of 32 total bits in an IPv4 address — the remaining bits identify individual hosts on that network. A /24 leaves 8 bits for hosts; a /16 leaves 16 bits.

Common Prefixes and What They Mean

Why Break a Network Into Subnets

Network Address, Broadcast Address, and Usable Range

Within any subnet, the first address is reserved as the network address and the last as the broadcast address — neither is assignable to a device. The usable host range is always two less than the total addresses in the subnet, which is why a /24 (256 total addresses) has 254 usable hosts, not 256.

Worked Example: Subnetting 192.168.10.77/26

Let's work through one address by hand, so you can check any calculator's result:

1. Host bits: 32 − 26 = 6 bits left for hosts.
2. Addresses per subnet: 2⁶ = 64 total addresses, so 64 − 2 = 62 usable hosts.
3. Subnet mask: /26 means the last octet has its top two bits set: 128 + 64 = 192, giving 255.255.255.192.
4. Which block? Subnets in the last octet start every 64: 0, 64, 128, 192. The host part of our address is 77, which falls in the block that starts at 64.
5. Result: network address 192.168.10.64, broadcast 192.168.10.127, usable range 192.168.10.65 – 192.168.10.126.

The "block size" trick — 256 minus the last non-255 octet of the mask (256 − 192 = 64) — is the fastest way to find subnet boundaries in your head.

Private Address Ranges

Home and office networks normally use the private ranges reserved by RFC 1918, which are not routed on the public internet: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. That's why your home router almost always hands out addresses like 192.168.1.x.

Common Mistakes to Avoid

• Forgetting the two reserved addresses. A /29 has 8 addresses but only 6 usable hosts.
• Assigning the network or broadcast address to a device, which causes hard-to-diagnose connectivity problems.
• Choosing a subnet with no room to grow. If a team has 25 devices today, a /27 (30 usable) is already tight; a /26 gives headroom.

Frequently Asked Questions

What about /31 and /32?

These are special cases. A /32 identifies a single host, and a /31 is allowed for point-to-point links between two routers (RFC 3021), where the two-reserved-addresses rule doesn't apply.

Does subnetting work the same way for IPv6?

The prefix idea is the same, but IPv6 addresses are 128 bits long and networks are usually /64. There's no broadcast address in IPv6, and address space is so large that conserving it is rarely a concern.