前缀长度 /22
/22 的子网掩码是 255.255.252.0
前 22 位是网络位，后 10 位是主机位。
- 子网掩码
- 255.255.252.0
- 二进制
- 11111111.11111111.11111100.00000000
- 通配符掩码
- 0.0.3.255
- 地址总数
- 1,024
- 可用主机数
- 1,022
- 块大小
- 4（第三个八位组）
怎么读这个数
掩码里连续 22 个 1 之后全是 0。写成点分十进制就是 255.255.252.0，第三个八位组是最后一个不满 255 的位置，子网就在这一组里按 4 递增。每个 /22 有 1,024 个地址，去掉网络地址和广播地址，剩 1,022 台主机可用。
通配符掩码 0.0.3.255 是子网掩码按位取反，ACL 和 OSPF 的 network 语句用它：0 表示这一位必须匹配，1 表示不关心。
能切出多少个
- 一个 /8 可以切成 16,384 个 /22（借 14 位）。
- 一个 /16 可以切成 64 个 /22（借 6 位）。
示例：10.0.0.0/16 切成 /22
| # | 网络地址 | 首个可用 | 末个可用 | 广播地址 | 可用主机数 |
|---|---|---|---|---|---|
| 1 | 10.0.0.0/22 | 10.0.0.1 | 10.0.3.254 | 10.0.3.255 | 1,022 |
| 2 | 10.0.4.0/22 | 10.0.4.1 | 10.0.7.254 | 10.0.7.255 | 1,022 |
| 3 | 10.0.8.0/22 | 10.0.8.1 | 10.0.11.254 | 10.0.11.255 | 1,022 |
| 4 | 10.0.12.0/22 | 10.0.12.1 | 10.0.15.254 | 10.0.15.255 | 1,022 |
| 5 | 10.0.16.0/22 | 10.0.16.1 | 10.0.19.254 | 10.0.19.255 | 1,022 |
| 6 | 10.0.20.0/22 | 10.0.20.1 | 10.0.23.254 | 10.0.23.255 | 1,022 |
| 7 | 10.0.24.0/22 | 10.0.24.1 | 10.0.27.254 | 10.0.27.255 | 1,022 |
| 8 | 10.0.28.0/22 | 10.0.28.1 | 10.0.31.254 | 10.0.31.255 | 1,022 |
| 9 | 10.0.32.0/22 | 10.0.32.1 | 10.0.35.254 | 10.0.35.255 | 1,022 |
| 10 | 10.0.36.0/22 | 10.0.36.1 | 10.0.39.254 | 10.0.39.255 | 1,022 |
| 11 | 10.0.40.0/22 | 10.0.40.1 | 10.0.43.254 | 10.0.43.255 | 1,022 |
| 12 | 10.0.44.0/22 | 10.0.44.1 | 10.0.47.254 | 10.0.47.255 | 1,022 |
| 13 | 10.0.48.0/22 | 10.0.48.1 | 10.0.51.254 | 10.0.51.255 | 1,022 |
| 14 | 10.0.52.0/22 | 10.0.52.1 | 10.0.55.254 | 10.0.55.255 | 1,022 |
| 15 | 10.0.56.0/22 | 10.0.56.1 | 10.0.59.254 | 10.0.59.255 | 1,022 |
| 16 | 10.0.60.0/22 | 10.0.60.1 | 10.0.63.254 | 10.0.63.255 | 1,022 |
| 17 | 10.0.64.0/22 | 10.0.64.1 | 10.0.67.254 | 10.0.67.255 | 1,022 |
| 18 | 10.0.68.0/22 | 10.0.68.1 | 10.0.71.254 | 10.0.71.255 | 1,022 |
| 19 | 10.0.72.0/22 | 10.0.72.1 | 10.0.75.254 | 10.0.75.255 | 1,022 |
| 20 | 10.0.76.0/22 | 10.0.76.1 | 10.0.79.254 | 10.0.79.255 | 1,022 |
| 21 | 10.0.80.0/22 | 10.0.80.1 | 10.0.83.254 | 10.0.83.255 | 1,022 |
| 22 | 10.0.84.0/22 | 10.0.84.1 | 10.0.87.254 | 10.0.87.255 | 1,022 |
| 23 | 10.0.88.0/22 | 10.0.88.1 | 10.0.91.254 | 10.0.91.255 | 1,022 |
| 24 | 10.0.92.0/22 | 10.0.92.1 | 10.0.95.254 | 10.0.95.255 | 1,022 |
| 25 | 10.0.96.0/22 | 10.0.96.1 | 10.0.99.254 | 10.0.99.255 | 1,022 |
| 26 | 10.0.100.0/22 | 10.0.100.1 | 10.0.103.254 | 10.0.103.255 | 1,022 |
| 27 | 10.0.104.0/22 | 10.0.104.1 | 10.0.107.254 | 10.0.107.255 | 1,022 |
| 28 | 10.0.108.0/22 | 10.0.108.1 | 10.0.111.254 | 10.0.111.255 | 1,022 |
| 29 | 10.0.112.0/22 | 10.0.112.1 | 10.0.115.254 | 10.0.115.255 | 1,022 |
| 30 | 10.0.116.0/22 | 10.0.116.1 | 10.0.119.254 | 10.0.119.255 | 1,022 |
| 31 | 10.0.120.0/22 | 10.0.120.1 | 10.0.123.254 | 10.0.123.255 | 1,022 |
| 32 | 10.0.124.0/22 | 10.0.124.1 | 10.0.127.254 | 10.0.127.255 | 1,022 |
| 33 | 10.0.128.0/22 | 10.0.128.1 | 10.0.131.254 | 10.0.131.255 | 1,022 |
| 34 | 10.0.132.0/22 | 10.0.132.1 | 10.0.135.254 | 10.0.135.255 | 1,022 |
| 35 | 10.0.136.0/22 | 10.0.136.1 | 10.0.139.254 | 10.0.139.255 | 1,022 |
| 36 | 10.0.140.0/22 | 10.0.140.1 | 10.0.143.254 | 10.0.143.255 | 1,022 |
| 37 | 10.0.144.0/22 | 10.0.144.1 | 10.0.147.254 | 10.0.147.255 | 1,022 |
| 38 | 10.0.148.0/22 | 10.0.148.1 | 10.0.151.254 | 10.0.151.255 | 1,022 |
| 39 | 10.0.152.0/22 | 10.0.152.1 | 10.0.155.254 | 10.0.155.255 | 1,022 |
| 40 | 10.0.156.0/22 | 10.0.156.1 | 10.0.159.254 | 10.0.159.255 | 1,022 |
| 41 | 10.0.160.0/22 | 10.0.160.1 | 10.0.163.254 | 10.0.163.255 | 1,022 |
| 42 | 10.0.164.0/22 | 10.0.164.1 | 10.0.167.254 | 10.0.167.255 | 1,022 |
| 43 | 10.0.168.0/22 | 10.0.168.1 | 10.0.171.254 | 10.0.171.255 | 1,022 |
| 44 | 10.0.172.0/22 | 10.0.172.1 | 10.0.175.254 | 10.0.175.255 | 1,022 |
| 45 | 10.0.176.0/22 | 10.0.176.1 | 10.0.179.254 | 10.0.179.255 | 1,022 |
| 46 | 10.0.180.0/22 | 10.0.180.1 | 10.0.183.254 | 10.0.183.255 | 1,022 |
| 47 | 10.0.184.0/22 | 10.0.184.1 | 10.0.187.254 | 10.0.187.255 | 1,022 |
| 48 | 10.0.188.0/22 | 10.0.188.1 | 10.0.191.254 | 10.0.191.255 | 1,022 |
| 49 | 10.0.192.0/22 | 10.0.192.1 | 10.0.195.254 | 10.0.195.255 | 1,022 |
| 50 | 10.0.196.0/22 | 10.0.196.1 | 10.0.199.254 | 10.0.199.255 | 1,022 |
| 51 | 10.0.200.0/22 | 10.0.200.1 | 10.0.203.254 | 10.0.203.255 | 1,022 |
| 52 | 10.0.204.0/22 | 10.0.204.1 | 10.0.207.254 | 10.0.207.255 | 1,022 |
| 53 | 10.0.208.0/22 | 10.0.208.1 | 10.0.211.254 | 10.0.211.255 | 1,022 |
| 54 | 10.0.212.0/22 | 10.0.212.1 | 10.0.215.254 | 10.0.215.255 | 1,022 |
| 55 | 10.0.216.0/22 | 10.0.216.1 | 10.0.219.254 | 10.0.219.255 | 1,022 |
| 56 | 10.0.220.0/22 | 10.0.220.1 | 10.0.223.254 | 10.0.223.255 | 1,022 |
| 57 | 10.0.224.0/22 | 10.0.224.1 | 10.0.227.254 | 10.0.227.255 | 1,022 |
| 58 | 10.0.228.0/22 | 10.0.228.1 | 10.0.231.254 | 10.0.231.255 | 1,022 |
| 59 | 10.0.232.0/22 | 10.0.232.1 | 10.0.235.254 | 10.0.235.255 | 1,022 |
| 60 | 10.0.236.0/22 | 10.0.236.1 | 10.0.239.254 | 10.0.239.255 | 1,022 |
| 61 | 10.0.240.0/22 | 10.0.240.1 | 10.0.243.254 | 10.0.243.255 | 1,022 |
| 62 | 10.0.244.0/22 | 10.0.244.1 | 10.0.247.254 | 10.0.247.255 | 1,022 |
| 63 | 10.0.248.0/22 | 10.0.248.1 | 10.0.251.254 | 10.0.251.255 | 1,022 |
| 64 | 10.0.252.0/22 | 10.0.252.1 | 10.0.255.254 | 10.0.255.255 | 1,022 |
练一下
给一个地址，前缀都是 /22。先自己算，再展开答案。
第 1 题：10.230.112.82/22 所在的网络、可用范围和广播地址
- 网络地址
10.230.112.0/22- 可用范围
10.230.112.1到10.230.115.254- 广播地址
10.230.115.255- 可用主机
- 1,022
第 2 题：10.129.44.91/22 所在的网络、可用范围和广播地址
- 网络地址
10.129.44.0/22- 可用范围
10.129.44.1到10.129.47.254- 广播地址
10.129.47.255- 可用主机
- 1,022
第 3 题：10.27.248.107/22 所在的网络、可用范围和广播地址
- 网络地址
10.27.248.0/22- 可用范围
10.27.248.1到10.27.251.254- 广播地址
10.27.251.255- 可用主机
- 1,022
第 4 题：10.130.52.236/22 所在的网络、可用范围和广播地址
- 网络地址
10.130.52.0/22- 可用范围
10.130.52.1到10.130.55.254- 广播地址
10.130.55.255- 可用主机
- 1,022
第 5 题：10.113.120.118/22 所在的网络、可用范围和广播地址
- 网络地址
10.113.120.0/22- 可用范围
10.113.120.1到10.113.123.254- 广播地址
10.113.123.255- 可用主机
- 1,022
想多练：按前缀分组的练习题。