Blocklist

A blocklist is a compiled collection of identifiers—such as IP addresses, domain names, email addresses, or user IDs—deemed unauthorized or undesirable and therefore blocked from access to a service. The term is often preferred over blacklist as a neutral description.

Blocklists are used to prevent abuse, protect systems, and conserve resources by preventing repeated unwanted activity. They can be managed by organizations, communities, or service providers and may be static, dynamically updated, or distributed via APIs.

Common applications include email filtering (DNS-based blackhole lists such as Spamhaus and SpamCop), network security (IP blocking in firewalls), web filtering (blocked URLs and domains), ad and content blocking, and moderation on social platforms.

Maintenance involves validating entries, updating with new information, and balancing accuracy with performance. Blocklists may suffer false positives, false negatives, evasion, or over-blocking, and raise privacy or censorship concerns.

Related concepts include allowlists (whitelists) and reputation-based systems. Blocklists are usually complemented by verification, user appeal processes, and transparent criteria to manage legitimate access while limiting abuse.