```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 prefix = 0;
  for (let i = 0; i < 4; i += 1) {
    const part = parts[i];
    if (!/^\d+$/.test(part)) {
      throw new Error("mask fields must be decimal digits");
    }
    const num = parseInt(part, 10);
    if (num > 255 || (i > 0 && num === 0)) {
      throw new Error("invalid octet value in mask");
    }

    for (let j = 7; j >= 0; j -= 1) {
      prefix += (num >> j) & 1;
    }
  }

  return prefix;
}
```