CyberKit

Subnetting

Work out what a CIDR block actually contains, and which special-use range an address falls in.

Drag octets and a prefix to build a block, with the network maths updating live. Foundational, about 20 minutes.

What you will be able to do

  • Convert between a prefix length, a netmask and a host count without a calculator (CIDR & VLAN Calculator)
  • Build the smallest CIDR block that contains a required set of addresses and excludes the rest
  • Read a network and broadcast address off a block and say which addresses are usable
  • Recognize the special-use IPv4 ranges on sight and say what each one means when it shows up in a log (IP & CIDR Reference)

Read first: IP & CIDR Reference.

Notes for teaching it
  • The live readout is the teaching surface here: have learners predict the network, broadcast and host count aloud BEFORE they drag, then check themselves against the panel.
  • Deliberately IPv4-only. IPv6 prefix maths is the same idea at a scale where the powers-of-two intuition stops helping, and mixing them in one drill teaches neither.
  • The special-use range questions are recall, not arithmetic, and tend to be where a class splits: consider running the IP & CIDR Reference on the projector for the first pass.

All questions

  1. A new lab segment needs a single CIDR block that exactly covers the 16 addresses from 10.30.6.0 through 10.30.6.15, no more and no fewer. Build the block.

  2. 100.64.55.10 falls inside 100.64.0.0/10 (Shared Address Space). What category does this block belong to?

  3. Team A's workstation pool must include 172.20.14.10 and 172.20.14.200. Team B owns the /24 blocks immediately before and after it. Build a single CIDR block that covers both of Team A's addresses without reaching into Team B's space.

  4. A firewall rule must match traffic from 10.50.8.64 and 10.50.8.95 only, not from 10.50.8.63, 10.50.8.96, or the unrelated address 10.50.9.1. Build the CIDR block.

  5. 169.254.10.5 falls inside 169.254.0.0/16 (Link Local (APIPA)). What category does this block belong to?

  6. A WAN link uses an RFC 3021 point-to-point IPv4 addressing scheme with just two host addresses and no separate network/broadcast address: 10.255.255.0 and 10.255.255.1. Build the smallest CIDR block containing both.

  7. An incident report references two hosts inside the same site allocation: 10.4.1.5 and 10.7.254.20. Build the smallest single CIDR block that contains both addresses.

  8. 192.0.2.55 falls inside 192.0.2.0/24 (Documentation (TEST-NET-1)). What category does this block belong to?

  9. An analyst wants a single rule that flags any traffic from IANA's benchmarking address space (RFC 2544), reserved for testing network-interconnect throughput so a benchmark never floods a real internet address. A benchmarking host lives at 198.18.0.1, another at 198.19.255.254. Build the CIDR block that covers the whole benchmarking range.

  10. A report cites hosts at 203.0.113.10 and 203.0.114.250, addresses in two adjacent /24s from this site's own documentation-range examples. Build the smallest single CIDR block that contains both addresses.

Answer key

Next: Network Reference Drill. All drills