题目要求：

countIP('10.0.0.0','10.0.0.50') //50

方式1：

function ipsBetween(start, end) {
  const ipLen = 4; 
  const step = 256;  

  let startNum = 0;
  let startList = start.split('.');
  for (let i = ipLen - 1; i >= 0; i--) {
    startNum += startList[i] * Math.pow(step, ipLen - i - 1);
  }

  let endNum = 0;
  let endList = end.split('.');
  for (let i = ipLen - 1; i >= 0; i--) {
    endNum += endList[i] * Math.pow(step, ipLen - i - 1);
  }

  return endNum - startNum;
}

方式2：

function ipsBetween(start, end){
let startNum=start.split('.').reduce((prev,cur,inx)=>prev+cur*Math.pow(256,3-inx),0);
let endNum=end.split('.').reduce((prev, cur, inx)=>prev+cur*Math. pow(256,3-inx),0);
    return endNum-startNum;
}
console.log(ipsBetween('10.0.0.0','10.0.0.50'))