What Is CIDR?
CIDR writes an IP prefix and length, such as 10.0.0.0/24. Learn how slash notation maps to masks, host counts, and why /24 is not the same as a single address.
CIDR is a prefix plus a length
CIDR (Classless Inter-Domain Routing) writes a network as address/prefix-length. The number after the slash is how many leading bits are the network. 192.168.1.0/24 means “the first 24 bits are fixed; the remaining 8 bits vary inside this block.”
The original class A/B/C rules are not required. RFC 4632 describes CIDR. For IPv4 vs IPv6 address shapes, see IPv4 vs IPv6.
A /24 example
192.168.1.0/24 is the same size as the old “Class C” idea, but the slash is what matters — not the first octet.
| Field | Value |
|---|---|
| CIDR | 192.168.1.0/24 |
| Subnet mask | 255.255.255.0 |
| Prefix bits | 24 |
| Host bits | 8 |
| Addresses in block | 256 (2^8) |
| Typical usable hosts | 192.168.1.1–192.168.1.254 |
| Broadcast (IPv4) | 192.168.1.255 |
IPv4 often reserves the network and broadcast addresses, so a /24 is commonly described as 254 usable hosts. /31 and /32 follow different operational rules (point-to-point and single host). Do not assume every prefix subtracts two addresses.
Common IPv4 prefix sizes
Smaller prefix numbers are larger networks. /16 is bigger than /24. /32 is a single IPv4 address.
| Prefix | Mask | Addresses |
|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 |
| /16 | 255.255.0.0 | 65,536 |
| /24 | 255.255.255.0 | 256 |
| /32 | 255.255.255.255 | 1 |
IPv6 also uses CIDR
IPv6 prefixes look like 2001:db8::/32. The slash still means “this many leading bits.” IPv6 does not use a broadcast address the way IPv4 does. A typical LAN advertisement is /64.
Where you will see CIDR
Security groups, VPC routes, nginx allow lists, Kubernetes NetworkPolicies, and “what IP range does this VPN use?” questions are usually CIDR. A single host in a firewall is often /32 (IPv4) or /128 (IPv6), not a bare address — though many UIs accept both.
Calculate a block instead of guessing
Paste 10.0.0.0/24 into the IP Address & CIDR Calculator to see network, mask, wildcard, and host range in your browser. To inspect a single address in binary or hex, use the IP Address Converter.
Frequently asked questions
- What does /24 mean in an IP address?
- The first 24 bits identify the network. For IPv4 that is a 256-address block, commonly written with mask 255.255.255.0.
- Is CIDR the same as a subnet mask?
- They describe the same split between network and host bits. CIDR uses a prefix length (/24). A mask uses dotted decimal (255.255.255.0). One can be converted into the other.
- What is the difference between 10.0.0.1 and 10.0.0.0/24?
- 10.0.0.1 is one host. 10.0.0.0/24 is a block of 256 addresses that includes 10.0.0.1. Firewall rules that should allow a LAN almost always need the prefix, not a single host.
- Does IPv6 use CIDR?
- Yes. IPv6 routing and allocation use prefix lengths such as /32, /48, and /64. The slash notation is the same idea as IPv4 CIDR.