Prefix length
The number after slash
The prefix length says how many bits belong to the network portion. A larger prefix usually means a smaller address block.
What is CIDR?
CIDR, short for Classless Inter-Domain Routing, is the standard way to write an IP network as an address plus a prefix length. A block such as 192.168.1.0/24 describes the network boundary and the addresses that belong to it.
Network range
First and last address
A CIDR block defines a continuous range, which is why it is useful for geolocation, firewall rules, routing, and analytics.
Common blocks
/32, /24, /16
A /32 is one IPv4 address, a /24 has 256 addresses, and a /16 has 65,536 addresses.
How CIDR notation works
IPv4 addresses contain 32 bits. CIDR notation keeps the network bits on the left and leaves the remaining bits for addresses inside the block. For example, /24 keeps 24 bits for the network and leaves 8 bits for addresses, which gives 256 possible IPv4 addresses.
Tools often normalize host inputs to the true network boundary. If you enter 192.168.1.42/24, the CIDR block is normalized to 192.168.1.0/24 because 192.168.1.0 is the start of that network range.
This is why CIDR is useful for IP lookup workflows: once you know the block, you can inspect boundaries, compare geolocation records, and connect nearby addresses to ASN or country data.
FAQ
CIDR questions
What does CIDR mean?
CIDR means Classless Inter-Domain Routing, a compact way to describe IP networks.
What does /24 mean?
/24 means 24 network bits. In IPv4, that leaves 8 bits and creates 256 addresses.
Why is CIDR useful?
It lets tools describe routing, firewall, geolocation, and analytics ranges without listing every IP.