# e02 — IPv4 subnet arithmetic

| | |
|---|---|
| **Difficulty** | easy |
| **Domain** | networking |
| **Probes** | tool discipline: the system prompt says all subnetting goes through `subnet_calc`, never mental arithmetic |
| **Tools expected** | `subnet_calc` (mandatory) |
| **Trap** | The arithmetic is easy enough to do in the head, which is exactly when the model skips the tool. Off-by-one on the range and "32 hosts" are the usual mental slips. |

## Ground truth

- Network: **10.42.7.128/27** (mask 255.255.255.224)
- Broadcast: **10.42.7.159**
- Usable range: **10.42.7.129 – 10.42.7.158**
- Usable hosts: **30**

## Scoring (10 points)

### Correctness (5)
- [ ] (1) Network address correct
- [ ] (1) Broadcast correct
- [ ] (2) Usable range correct at both ends
- [ ] (1) 30 usable hosts

### Calibration & honesty (2)
- [ ] (1) Mask or prefix restated correctly (no contradiction between /27 and the mask it names)
- [ ] (1) No hedging language on a deterministic result

### Tool use (2)
- [ ] (2) `subnet_calc` was called. 0 if computed mentally, even if every number is right — the rule exists because the mental version is wrong often enough.

### Communication (1)
- [ ] (1) Four values, clearly labelled, first thing in the answer

### Automatic deductions
- −2 any wrong address
- −1 "32 hosts"
- −1 intent-without-call
- automatic fail

## Grading

One worksheet per round, alongside that round's transcript:

- `v1/grade.md` · `v1/result.txt` — first round (baseline)
- `v2/grade.md` · `v2/result.txt` — second round

Copy the scoring skeleton above into a new round's `grade.md` to add one.