文章内容
2019/10/12 20:05:29,作 者: 黄兵
CIDR，子网掩码和可用IP地址快速参考指南（备忘单）
快速参考子网和CIDR指南。
什么是 /28？/26 可以使用几个IP？好吧，请参见下表！
可用IP是IP总数减去网络IP和广播IP。
CIDR | 子网掩码 | IP总数 | 可用IP |
| / 32 | 255.255.255.255 | 1个 | 1个 |
| / 31 | 255.255.255.254 | 2 | 0 |
| / 30 | 255.255.255.252 | 4 | 2 |
| / 29 | 255.255.255.248 | 8 | 6 |
| / 28 | 255.255.255.240 | 16 | 14 |
| / 27 | 255.255.255.224 | 32 | 30 |
| / 26 | 255.255.255.192 | 64 | 62 |
| / 25 | 255.255.255.128 | 128 | 126 |
| / 24 | 255.255.255.0 | 256 | 254 |
| / 23 | 255.255.254.0 | 512 | 510 |
| / 22 | 255.255.252.0 | 1024 | 1022 |
| / 21 | 255.255.248.0 | 2048 | 2046 |
| / 20 | 255.255.240.0 | 4096 | 4094 |
| / 19 | 255.255.224.0 | 8192 | 8190 |
| / 18 | 255.255.192.0 | 16,384 | 16,382 |
| / 17 | 255.255.128.0 | 32,768 | 32,766 |
| / 16 | 255.255.0.0 | 65,536 | 65,534 |
| / 15 | 255.254.0.0 | 131,072 | 131,070 |
| / 14 | 255.252.0.0 | 262,144 | 262,142 |
| / 13 | 255.248.0.0 | 524,288 | 524,286 |
| / 12 | 255.240.0.0 | 1,048,576 | 1,048,574 |
| / 11 | 255.224.0.0 | 2,097,152 | 2,097,150 |
| / 10 | 255.192.0.0 | 4,194,304 | 4,194,302 |
| / 9 | 255.128.0.0 | 8,388,608 | 8,388,606 |
| / 8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
| / 7 | 254.0.0.0 | 33,554,432 | 33,554,430 |
| / 6 | 252.0.0.0 | 67,108,864 | 67,108,862 |
| / 5 | 248.0.0.0 | 134,217,728 | 134,217,726 |
| / 4 | 240.0.0.0 | 268,435,456 | 268,435,454 |
| / 3 | 224.0.0.0 | 536,870,912 | 536,870,910 |
| / 2 | 192.0.0.0 | 1,073,741,824 | 1,073,741,822 |
| / 1 | 128.0.0.0 | 2,147,483,648 | 2,147,483,646 |
| / 0 | 0.0.0.0 | 4,294,967,296 | 4,294,967,294 |
A类IP地址的默认子网掩码为255.0.0.0（由于255相当于二进制的8位1，所以也缩写成“/8”，表示网络号占了8位）;
B类的为255.255.0.0（/16）;
C类的为255.255.255.0(/24)
/30就是255.255.255.252
/32就是255.255.255.255
参考资料：CIDR, Subnet Masks, and Usable IP Addresses Quick Reference Guide (Cheat Sheet)
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