https://blog.gongquiz.com/2022/11/13/%eb%a9%80%ed%8b%b0%ec%ba%90%ec%8a%a4%ed%8a%b8multicast%ec%97%90-%ec%82%ac%ec%9a%a9%eb%90%98%eb%8a%94-ip-class%eb%8a%94-%eb%84%a4%ed%8a%b8%ec%9b%8c%ed%81%ac%ea%b4%80%eb%a6%ac%ec%82%ac-2%ea%b8%89/