Design an IP filter using CIDR rules

Q: Design an IP filter using CIDR rules

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Question

Explain CIDR notation with a couple of concrete examples. Show how to convert a prefix like 192.168.0.0/16 into an inclusive 32-bit integer range and a subnet mask. Then, design a data structure for a firewall rule set: each rule is (prefix: CIDR string, action: 'allow' | 'deny', priority: integer). Implement addRule(rule), removeRule(ruleId), and query(ipAddress) -> action. Specify the matching semantics when prefixes overlap (support both first-match-by-priority and longest-prefix-match), target time/space complexity, and how you would handle up to 10^6 rules and 10^5 queries per second. Follow-up: the query is now a CIDR block instead of a single IP. Add operations overlap(queryPrefix) -> boolean, coveredByAllow(queryPrefix) -> boolean, coveredByDeny(queryPrefix) -> boolean, and listConflictingRules(queryPrefix) -> [ruleIds]. Which data structures would you choose to support efficient range and overlap queries, and how would your design extend to IPv6?

Design an IP filter using CIDR rules

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