https://itmanual.net/%ec%9c%88%eb%8f%84%ec%9a%b0-%eb%84%a4%ed%8a%b8%ec%9b%8c%ed%81%ac-%eb%aa%85%eb%a0%b9%ec%96%b4-ping-nslookup-%ec%82%ac%ec%9a%a9%eb%b2%95/