What is CIDR Block?

A CIDR Block (Classless Inter-Domain Routing) is a method of allocating IP addresses. It is represented by an IP address followed by a slash and a prefix length (e.g., 10.0.0.0/24). The prefix length indicates how many bits of the address are fixed for the network, leaving the remaining bits for individual host IPs.

In the context of web scraping, CIDR blocks represent the grouping of your proxy infrastructure. Because IP addresses are often purchased and assigned in contiguous blocks, all IPs in a /24 subnet are usually owned by the same entity and located in the same datacenter.

Modern Web Application Firewalls (WAFs) do not evaluate IP reputation in isolation. They aggregate risk scores at the /24 (IPv4) or /64 (IPv6) subnet level. If a scraper hits a target using 192.168.1.10 and gets flagged, the WAF increases the threat score for the entire 192.168.1.0/24 block.

If multiple IPs in that block exhibit bot-like behavior, the WAF will deploy a subnet-level block. Any subsequent request from a completely fresh, unused IP in that same CIDR block will be instantly rejected with a 403 or a CAPTCHA.

Datacenter proxies are highly vulnerable to CIDR blocking because they are provisioned in large, contiguous blocks. A provider might sell you 1,000 IPs, but they all sit within four /24 subnets. Banning them is trivial for a WAF.

Residential proxies, by contrast, are highly fragmented. Because they are sourced from individual consumer devices, a pool of 1,000 residential IPs might span 900 different /24 CIDR blocks across dozens of ISPs. This fragmentation makes subnet-level blocking nearly impossible without causing massive collateral damage to real human users.

We treat subnet diversity as a primary routing metric. Our proxy orchestrator maps the CIDR topology of our entire fleet. When scheduling a high-concurrency crawl, we enforce strict cross-CIDR scattering—ensuring that no two concurrent requests to the same target originate from the same /24 block.

If we detect elevated challenge rates on a specific IP, we don't just rotate the IP; we quarantine the entire /24 subnet for that specific target domain, allowing its reputation to recover before it triggers a hard ban.

If you buy shared proxies, you share CIDR blocks with other customers. Even if your scraping pipeline is perfectly tuned, respectful of rate limits, and uses flawless browser fingerprints, your IPs can still be blocked.

Why? Because another customer using the same proxy provider might be running a brute-force attack against your target using different IPs in the same /24 block. The WAF bans the subnet, and your pipeline goes down as collateral damage. This is why dedicated IPs or highly fragmented residential pools are required for mission-critical data extraction.