CIDR Notation and Subnetting Explained
CIDR (Classless Inter-Domain Routing) notation describes an IP address range as a base address plus a prefix length, like 192.168.1.0/24, where the prefix length says how many leading bits are fixed as the "network" portion, leaving the rest available for individual hosts.
Last updated: August 29, 2026
What does a CIDR block like /24 actually mean?
In 192.168.1.0/24, the /24 means the first 24 bits (three full octets) identify the network, leaving the last 8 bits (256 possible values) for hosts. A smaller prefix (like /16) means fewer fixed network bits and a much larger address range; a larger prefix (like /28) means more fixed bits and a smaller range.
Network, broadcast, and usable host range
Within an IPv4 subnet, the first address is reserved as the network address (identifies the subnet itself) and the last is reserved as the broadcast address (sends to every host on the subnet); neither is assignable to an individual device. A /24 has 256 total addresses, but only 254 are usable host addresses once those two are set aside.
Common prefix lengths, quick reference
| Prefix | Total addresses | Usable hosts (IPv4) |
|---|---|---|
| /30 | 4 | 2 |
| /29 | 8 | 6 |
| /28 | 16 | 14 |
| /27 | 32 | 30 |
| /26 | 64 | 62 |
| /24 | 256 | 254 |
| /16 | 65,536 | 65,534 |
How is IPv6 subnetting different from IPv4?
IPv6 doesn't reserve a broadcast address at all; multicast handles that role instead. A subnet's usable range isn't reduced by two the way IPv4's is. IPv6 subnets are also conventionally sized at /64 regardless of how many hosts will actually be on them, since the address space is large enough that conserving addresses isn't the driving concern it is in IPv4.
Get the network address, broadcast address, netmask, wildcard mask and usable host range/count from any CIDR block, for both IPv4 and IPv6.
Frequently asked questions
Why do we lose 2 usable addresses per IPv4 subnet?
The first address in the range is reserved to identify the subnet itself (the network address) and the last is reserved for broadcasting to every host on it; neither can be assigned to an individual device.
What's the difference between a subnet mask and CIDR notation?
They're two ways of writing the same thing. A subnet mask like 255.255.255.0 and a CIDR prefix like /24 both describe "the first 24 bits are the network portion." CIDR notation is just more compact.
How many usable hosts does a /27 give me?
A /27 has 32 total addresses; subtracting the network and broadcast addresses leaves 30 usable host addresses.
Do IPv6 subnets have a broadcast address?
No. IPv6 removed broadcast entirely in favor of multicast, so IPv6 subnet math doesn't need to reserve an address for it the way IPv4 does.