```typescript
export function maskFromPrefix(prefix: number): string {
  if (!Number.isInteger(prefix) || prefix < 0 || prefix > 32) {
    throw new Error("prefix must be an integer from 0 to 32");
  }
  const octets: number[] = [];
  for (let slot = 0; slot < 4; slot += 1) {
    const ones = Math.min(8, Math.max(0, prefix - slot * 8));
    octets.push(256 - 2 ** (8 - ones));
  }
  return octets.join(".");
}

export function prefixFromMask(mask: string): number {
  if (typeof mask !== "string") {
    throw new Error("mask must be a string");
  }
  const parts = mask.split(".");
  if (parts.length !== 4) {
    throw new Error("mask must have exactly four dot-separated fields");
  }
  let prefix = 0;
  for (const part of parts) {
    const num = parseInt(part, 10);
    if (!Number.isInteger(num) || num < 0 || num > 255 || (num > 0 && part.startsWith("0"))) {
      throw new Error("mask fields must be decimal digits without leading zeros");
    }
    if (num === 0) {
      prefix += 8;
    } else {
      while (num & 128) {
        prefix += 1;
        num <<= 1;
      }
      if ((num & 254) !== 0) {
        throw new Error("mask ones must not be broken by a zero");
      }
    }
  }
  return prefix;
}
```