https://blog.gongquiz.com/2021/01/18/%ec%84%9c%eb%b8%8c%eb%84%b7-%eb%a7%88%ec%8a%a4%ed%81%acsubnet-mask%ec%97%90-%eb%8c%80%ed%95%9c-%ec%84%a4%eb%aa%85-%ec%a4%91-%ec%98%ac%eb%b0%94%eb%a5%b8-%ea%b2%83%ec%9d%80-%eb%84%a4%ed%8a%b8/