What is CIDR?
CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and defining IP routing policies. It replaced the older classful network addressing scheme (Class A/B/C) and allows for more flexible allocation of IP address space.
CIDR Notation
CIDR notation combines an IP address with a suffix indicating the number of bits in the network prefix:
192.168.1.0/24— 256 addresses (192.168.1.0 to 192.168.1.255)10.0.0.0/8— 16,777,216 addresses (entire 10.x.x.x block)172.16.0.0/12— 1,048,576 addresses
The number after the slash (/) indicates how many bits of the IP address are used for the network portion. The remaining bits identify individual hosts.
Common CIDR Blocks
| CIDR Suffix | Addresses | Subnet Mask | Typical Use |
|---|---|---|---|
/32 |
1 | 255.255.255.255 | Single host |
/24 |
256 | 255.255.255.0 | Small network |
/16 |
65,536 | 255.255.0.0 | Medium network |
/8 |
16,777,216 | 255.0.0.0 | Large ISP block |
Reserved CIDR Ranges
| Range | Purpose |
|---|---|
10.0.0.0/8 |
Private (RFC 1918) |
172.16.0.0/12 |
Private (RFC 1918) |
192.168.0.0/16 |
Private (RFC 1918) |
127.0.0.0/8 |
Loopback |
0.0.0.0/0 |
Default route (all IPs) |
Search on Zondex
Use the cidr: filter to search by CIDR range:
cidr:203.0.113.0/24— search within a /24 network rangecidr:8.8.8.0/24— find hosts in Google's DNS networkcidr:1.1.1.0/24— find hosts in Cloudflare's DNS network