CIDR
Definition
Classless Inter-Domain Routing - method for allocating IP addresses and routing internet traffic efficiently, optimizing network performance.
Use Cases
- Cloudflare: Publishing IP ranges so customers can allowlist Cloudflare edge traffic and so networks can route traffic efficiently to Cloudflare. — Cloudflare publishes its IPv4 and IPv6 CIDR blocks (route prefixes) for its edge network. Customers and partners use these CIDR ranges in firewall rules, WAF allowlists, and routing policies to permit or prioritize traffic to/from Cloudflare. (Simplifies network policy management (allowlisting by prefix rather than individual IPs) and supports scalable routing and filtering as Cloudflare grows.)
- Amazon Web Services (AWS): Letting customers configure VPC and subnet IP ranges for workloads and segment environments (prod/dev) without overlapping networks. — AWS customers define a VPC CIDR block (for example, 10.0.0.0/16) and then create multiple subnet CIDR blocks within it (for example, 10.0.1.0/24, 10.0.2.0/24). They use route tables and security groups/NACLs referencing these CIDRs to control traffic flows. (Enables predictable IP planning, easier segmentation, and safer connectivity to on-prem networks and other VPCs by avoiding overlapping address space.)
- Google: Aggregating routes to reduce routing table size and improve internet routing efficiency. — Large networks commonly advertise aggregated CIDR prefixes (route summarization) rather than many small, specific routes. This reduces the number of routes that upstream routers must store and process, improving scalability and stability. (More efficient routing operations, smaller routing tables, and improved manageability compared with advertising many individual network routes.)
Frequently Asked Questions
- What's the difference between CIDR and a subnet mask?
- They represent the same idea (how many bits are the network portion vs. host portion), but they’re written differently. A subnet mask uses dotted-decimal (for example, 255.255.0.0). CIDR uses a prefix length (for example, /16). So 10.0.0.0/16 is equivalent to a subnet mask of 255.255.0.0.
- When should I use CIDR?
- Use CIDR whenever you define or reason about an IP range: creating a cloud VPC/VNet/VCN, creating subnets, setting firewall or security rules (allow/deny by IP range), configuring VPN/peering routes, or planning IP space to avoid overlaps between networks.
- How much does CIDR cost?
- CIDR itself is a notation/standard and has no direct cost. Costs come from the cloud resources that use CIDR-defined networks (for example, VPC/VNet components, NAT gateways, VPNs, load balancers, or data transfer). Some providers may also charge for scarce public IPv4 addresses, but that is separate from CIDR notation.
Category: networking
Difficulty: advanced
Related Terms
See Also