```ts
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 countLeadingOnes = 0;
  for (const part of parts) {
    if (!/^\d+$/.test(part)) {
      throw new Error("field must hold decimal digits");
    }
    if (part.startsWith('0') && part !== '0') {
      throw new Error("field with leading zero other than 0 itself");
    }
    const num = parseInt(part, 10);
    if (num > 255) {
      throw new Error("octet above 255");
    }
    countLeadingOnes += num.toString(2).split('0').length - 1;
  }
  return countLeadingOnes;
}
```