---
title: "What is CIDR (Classless Inter-Domain Routing)?"
url: "https://lookvalid.com/glossary/cidr.md"
canonical_url: "https://lookvalid.com/glossary/cidr"
last_updated: "2026-10-23"
type: "conceptual"
summary: "CIDR notation writes a block of IP addresses as a starting address and a prefix length, such as 192.0.2.0/24."
related: ["https://lookvalid.com/glossary/asn.md", "https://lookvalid.com/glossary/datacenter-ip.md", "https://lookvalid.com/glossary/reverse-dns.md", "https://lookvalid.com/products/ip.md"]
---

# What is CIDR (Classless Inter-Domain Routing)?

CIDR notation writes a block of IP addresses as a starting address and a prefix length, such as 192.0.2.0/24.

The number after the slash says how many leading bits are fixed. A /24 holds 256 addresses, a /16 holds 65,536, and a smaller number means a larger block. IPv6 uses the same notation with longer prefixes.

Lists of cloud, proxy and abuse addresses are published as CIDR blocks, so checking whether an address is covered is a prefix match.

You can turn a prefix into a range by counting host bits. An IPv4 address has 32 bits, so a /24 leaves 8 host bits and 2^8 = 256 addresses. Published lists often hold overlapping blocks, so a lookup should return the most specific match, which is the longest prefix, and not just the first one it finds.

## Related

- [What is ASN (Autonomous System Number)?](https://lookvalid.com/glossary/asn.md)
- [What is Datacenter IP?](https://lookvalid.com/glossary/datacenter-ip.md)
- [What is Reverse DNS (pointer (PTR) record)?](https://lookvalid.com/glossary/reverse-dns.md)
- [IP Intelligence API](https://lookvalid.com/products/ip.md)