network submask cal

 https://damaha-labo.site/converter/zh-Hant/IT/CalcSubnetMask

https://www.iptp.net/zh_HK/iptp-tools/ip-calculator/

https://david50.pixnet.net/blog/post/45215556-%5B%E7%AD%86%E8%A8%98%5Dipv4-submask%E8%A8%88%E7%AE%97

http://webtest.ccivs.cyc.edu.tw/net/new1.htm

https://ithelp.ithome.com.tw/questions/10153316