CIDR Range Calculator
CIDR Range Calculator
Practical CIDR notation skills — the kind you need when a vendor hands you eight /24 blocks and you know there’s a better way.
What You’ll Practice
Phase 1 — Decode: Given a CIDR block, identify the start address, end address, subnet mask, and usable host count. Includes blocks where the written IP isn’t the actual network address.
Phase 2 — Classify: Firewall ACL simulation. Does a given IP fall within a CIDR block? Boundary cases included.
Phase 3 — Aggregate: The real-world skill. Given a list of individual blocks, collapse them into the minimum CIDR set. Alignment rules enforced — the starting address must be divisible by the block size.
Phase 4 — Find Redundancies: Audit a messy whitelist and flag entries that are already covered by broader blocks in the same list.
Why This Matters
Every firewall rule, every cloud security group, every vendor IP whitelist uses CIDR notation. Misreading a range means either blocking legitimate traffic or leaving holes in your perimeter. The aggregation skill specifically saves you from maintaining bloated ACLs that are harder to audit and easier to misconfigure.
MISSION
Master CIDR notation through four escalating challenges. Start by decoding ranges, then evaluate firewall rules, consolidate vendor whitelists, and audit ACLs for redundant entries.
Every question maps to real-world network administration tasks you will face on the job and on the exam.