Subnetting Practice

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What Tier 1 Covers

Most problems give you an address and a prefix, like 192.168.23.138/21, and ask for one of:

• Network address — the first address in the block. Line the mask up under the address in binary and AND them. Every host bit becomes 0.
• Broadcast address — the last address in the block. Start from the network address and set every host bit to 1.
• Usable host count — a /21 leaves 32 − 21 = 11 host bits, so 211 = 2048 total addresses. Subtract two (network + broadcast) to get 2046 usable.
• Usable host range — network address + 1 through broadcast address − 1.
• Subnet mask — the first prefix bits set to 1, the rest 0, read as four decimals (a /21 is 255.255.248.0).
• Wildcard mask — the subnet mask with every bit flipped (/21 → 0.0.7.255).
• CIDR from a mask — count the leading 1 bits of the mask.

Prefixes run from /8 to /30. The ? next to a question gives the method; Show worked solution shows the binary lined up.

Tier 2 — Applied

Switch the difficulty selector to Tier 2 for the questions that come up in real design work:

• Which subnet — a parent block is split into equal subnets; find the one a given host falls in.
• Borrow bits — borrow b bits from a prefix: how many subnets (2^b), and how many usable hosts per subnet.
• Nth subnet — the network address of the K-th equal-size subnet of a block.
• Same subnet? — do two addresses share a given /n.
• Prefix for N hosts — the largest subnet (smallest prefix) that still fits a required number of usable hosts.

Tier 3 — Design

• VLSM — carve one block into right-sized subnets for a list of host requirements, largest first, contiguous from the base.
• Route summarisation — the single shortest prefix that covers a set of networks.
• IPv6 subnet count — how many child prefixes fit in a parent (for example, /64s in a /48).
• IPv6 compression — write an address in shortest RFC 5952 form, or expand it back to all eight groups.

Why Practise This By Hand

Subnetting is on the CCNA and CompTIA Network+ exams without a calculator, and it comes up constantly in real network design and troubleshooting — working out whether two hosts are on the same subnet, how many addresses a VLAN needs, or which range a firewall rule should cover. Doing it on paper until it is fast is the point.

Need to check a full breakdown of any block? Use the Subnet Calculator. New to the topic? Start with Understanding Subnets and IP Addresses or What is CIDR.