# Subnetting Drills

Subnetting is arithmetic, and arithmetic becomes fast with ten minutes a day. This note is the method, then the drills, then the answers.

## The method that scales to /30

You need three numbers: **block size**, **network address**, **broadcast address**.

1. **Block size** = 256 − (the mask octet you are in). For /26 the mask octet is 224, so block size = 32.
2. **Networks** start at multiples of the block size in the interesting octet.
3. **Broadcast** = next network − 1.
4. **Usable hosts** = broadcast − network − 1.

| Prefix | Mask octet | Block size | Usable |
|---|---|---|---|
| /25 | 128 | 128 | 126 |
| /26 | 192 | 64 | 62 |
| /27 | 224 | 32 | 30 |
| /28 | 240 | 16 | 14 |
| /29 | 248 | 8 | 6 |
| /30 | 252 | 4 | 2 |
| /31 | 254 | 2 | 2 (point-to-point, no broadcast) |
| /32 | 255 | 1 | 1 (host route) |

Memorise this table and most questions collapse into one subtraction.

## Worked example, slowly

**Which subnet is 192.168.10.77/27 in, and what are its bounds?**

- /27 → mask 255.255.255.224 → block size 32.
- Multiples of 32: 0, 32, 64, **96**.
- 77 falls between 64 and 96 → network **192.168.10.64**.
- Broadcast = 96 − 1 = **95**.
- Usable = 65–94 (30 addresses).

## Drills

Answer before scrolling. Aim for under 20 seconds each.

1. Network and broadcast for `10.0.5.130/25`?
2. How many usable hosts in `/28`?
3. Is `172.16.4.200/26` in the same subnet as `172.16.4.190/26`?
4. Split `192.168.0.0/24` into four equal subnets. List them.
5. Smallest prefix that holds 50 hosts?
6. Smallest prefix that holds 500 hosts?
7. Point-to-point link, minimal waste: which prefix?
8. `203.0.113.0/24` — network and broadcast of the 6th /28?
9. Two hosts, `10.1.1.10/24` and `10.1.2.10/24`: same subnet?
10. Summarise `10.8.0.0/24` through `10.11.0.0/24` into one prefix.

## Answers

1. Block 128 → network `10.0.5.128`, broadcast `10.0.5.255`.
2. 16 − 2 = **14**.
3. Block 64 → 200 is in 192–255, 190 is in 128–191. **Different** subnets.
4. /26 → `192.168.0.0`, `.64`, `.128`, `.192`, each with 62 usable.
5. 50 hosts needs 6 host bits (62) → **/26**.
6. 500 needs 9 host bits (510) → **/23**.
7. **/31** (RFC 3021), or /30 if the platform does not support /31.
8. /28 block 16; networks at 0,16,32,48,64,80 → 6th is `203.0.113.80`, broadcast `.95`.
9. **No** — /24 boundaries fall between 10.1.1.x and 10.1.2.x.
10. **10.8.0.0/22** (covers 10.8.0.0–10.11.255.255).

## The mistakes that matter in practice

| Mistake | Consequence |
|---|---|
| Gateway outside the host's subnet | Host ARPs for an address it cannot reach, or sends to the wrong next hop |
| /24 assumed when the mask is /25 | Half the hosts are unreachable and it looks intermittent |
| Two subnets with the same CIDR | Routing becomes ambiguous; one side is silently unreachable |
| Summarising wider than the addresses present | A blackhole for the gap |
| Forgetting the network and broadcast | Off-by-two in capacity planning, occasionally in DHCP scopes |
| Overlapping ranges across VPN/VPC | Blocks the tunnel entirely; requires renumbering |

## IPv6, briefly

No broadcast, no NAT, and the arithmetic is hexadecimal. The equivalents:

- **/64** — one subnet, always; SLAAC requires it.
- **/56** — a typical site allocation (256 subnets).
- **/48** — a typical organisation allocation (65,536 subnets).
- **/127** — point-to-point links.
- Subnetting is on the fourth hextet boundary; you almost never subnet below /64.

The habit that transfers: think in **prefix lengths and boundaries**, not in dotted decimal.

## Teaching hook

**The ruler analogy.** A prefix is a mark on a ruler; everything before the mark is fixed (network), everything after is free (hosts). The block size is the distance between marks. Once learners see the ruler, they stop doing binary.

Drill format that works: ten questions, 60 seconds, then answers read aloud. Repeat daily for a week; it becomes automatic.

## Related Notes

- [[Foundations/Addressing-Subnetting-CIDR.md]] — the reference tables
- [[Labs/Home-Lab-Setup.md]] — apply it to lab 1
- [[Routing/Route-Selection-And-Metrics.md]] — longest prefix match
- [[Cloud-Modern-Networking/Cloud-Networking-Patterns.md]] — why the plan matters more in cloud