192.168.1.10/24 is how most developers see an IP address every day, and most couldn't explain what exactly the /24 means without stopping to think. Here's the cheat sheet.
What CIDR notation means
The number after the slash (the prefix) tells you how many of the 32 bits in the IPv4 address are "fixed" to identify the network; the rest are free to number hosts within it. /24 fixes the first 24 bits (the first three octets) and leaves 8 bits free — 256 possible addresses in that subnet.
The four addresses that matter
Given an IP and a prefix, four values follow mathematically:
- Network mask: the prefix expressed as an IP (
/24→255.255.255.0). - Network address: the IP with all host bits set to 0 — identifies the subnet, not an actual host.
- Broadcast: the IP with all host bits set to 1 — messages to this address reach every host on the subnet.
- Usable hosts:
2^(32-prefix) - 2(subtracting the network and broadcast addresses, which can't be assigned to a device).
For /24: 2⁸ - 2 = 254 usable hosts. For /28 (common in small office subnets): 2⁴ - 2 = 14 hosts.
The special case of /31 and /32
A /32 has no host bits at all — it's a single exact address (used for specific routes). A /31, per RFC 3021, is used on point-to-point links (say, between two routers) and reserves neither a network nor a broadcast address — both of its 2 addresses are usable, an exception to the "-2" rule that surprises anyone seeing it for the first time.
Private addresses (RFC 1918)
If your IP starts with 10.x.x.x, 172.16.x.x–172.31.x.x or 192.168.x.x, you're on a private network — not directly routable on the internet, reserved for local networks. Everything else (except loopback 127.x.x.x and a few reserved ranges) is potentially public.
Calculate it without doing binary math by hand
The subnet calculator takes any IP and prefix and gives you the mask, network, broadcast, usable hosts, and whether the IP is private — with binary and hexadecimal representations if you need them for debugging. If you also work with JWTs in the same stack, the JWT decoder does the same kind of "turn a technical format into something readable" work for tokens.