```ts
export function maskFromPrefix(prefix: number): string {
  if (!Number.isInteger(prefix) || prefix < 0 || prefix > 32) {
    throw new Error('Invalid prefix');
  }
  const ones = '1'.repeat(prefix);
  const zeros = '0'.repeat(32 - prefix);
  return [ones, zeros].join('.').padEnd(4, '0').replace(/(\d{8})/g, '$1.');
}

export function prefixFromMask(mask: string): number {
  if (typeof mask !== 'string') {
    throw new Error('Invalid mask');
  }
  const parts = mask.split('.');
  if (parts.length !== 4) {
    throw new Error('Invalid mask format');
  }
  let count = 0;
  for (const part of parts) {
    if (!/^\d+$/.test(part)) {
      throw new Error('Invalid mask field');
    }
    const num = parseInt(part, 10);
    if (num > 255 || (part.length > 1 && part.startsWith('0'))) {
      throw new Error('Invalid mask field');
    }
    if (num === 0) {
      break;
    }
    count += 8 - Math.floor(Math.log2(256 - num));
  }
  return count;
}
```