Networking · Network & IP
Subnetting 101: CIDR, networks, broadcasts, and host ranges
CIDR notation looks cryptic — 10.0.0.0/8, 192.168.1.0/24, 172.16.0.0/12 — but each piece has a clear meaning. The number after the slash is the prefix length, which tells you how many bits of the IP address are the network part and how many are the host part. From that, you can derive every other field.
TL;DR
A subnet calculator takes a CIDR like 192.168.1.0/24 and gives you back the network address, broadcast, host range, wildcard, and IP class. Here is what each piece means.
Reading a CIDR
192.168.1.0/24 means the first 24 bits are the network, the remaining 8 bits are the host. 24 bits = 3 bytes = the first three octets of the IP. So 192.168.1.0 is the network address, and the 8 host bits give 2^8 = 256 possible addresses (0 through 255). Of those 256, two are reserved — the network itself (192.168.1.0) and the broadcast (192.168.1.255). That leaves 254 usable host addresses.
Where the common masks come from
/8 = 255.0.0.0 = 16,777,214 usable hosts. /16 = 255.255.0.0 = 65,534 hosts. /24 = 255.255.255.0 = 254 hosts. /30 = 255.255.255.252 = 2 hosts (used for point-to-point links between routers). /32 = 255.255.255.255 = 1 host (a single IP). The relationship is exponential: each prefix-length step doubles the host count.
IP classes (the historical system)
Class A was 0.0.0.0/8 to 127.255.255.255 (huge blocks for governments and big companies). Class B was 128.0.0.0/16 to 191.255.255.255 (mid-size networks). Class D is 224.0.0.0/4 (multicast). Class E is 240.0.0.0/4 (experimental, reserved). Classful networking was replaced by CIDR in 1993, but the terminology persists and Class C is still used colloquially to mean "a /24 block".
Wildcard masks vs subnet masks
The subnet mask says which bits are the network (255 for network bits, 0 for host bits). The wildcard mask is the inverse — it says which bits to ignore when matching (0 for network bits, 255 for host bits). You will see wildcard masks in Cisco ACL config and in firewall rules. The math is identical: a /24 has subnet mask 255.255.255.0 and wildcard mask 0.0.0.255.
Try it →
Run the Subnet calculator tool on sitetrace.it.com — paste a value and get an instant answer.
Open Subnet calculator