前缀长度 /15
/15 的子网掩码是 255.254.0.0
前 15 位是网络位，后 17 位是主机位。
- 子网掩码
- 255.254.0.0
- 二进制
- 11111111.11111110.00000000.00000000
- 通配符掩码
- 0.1.255.255
- 地址总数
- 131,072
- 可用主机数
- 131,070
- 块大小
- 2（第二个八位组）
怎么读这个数
掩码里连续 15 个 1 之后全是 0。写成点分十进制就是 255.254.0.0，第二个八位组是最后一个不满 255 的位置，子网就在这一组里按 2 递增。每个 /15 有 131,072 个地址，去掉网络地址和广播地址，剩 131,070 台主机可用。
通配符掩码 0.1.255.255 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 128 个 /15（借 7 位）。
示例：10.0.0.0/8 切成 /15
| # | 网络地址 | 首个可用 | 末个可用 | 广播地址 | 可用主机数 |
|---|---|---|---|---|---|
| 1 | 10.0.0.0/15 | 10.0.0.1 | 10.1.255.254 | 10.1.255.255 | 131,070 |
| 2 | 10.2.0.0/15 | 10.2.0.1 | 10.3.255.254 | 10.3.255.255 | 131,070 |
| 3 | 10.4.0.0/15 | 10.4.0.1 | 10.5.255.254 | 10.5.255.255 | 131,070 |
| 4 | 10.6.0.0/15 | 10.6.0.1 | 10.7.255.254 | 10.7.255.255 | 131,070 |
| 5 | 10.8.0.0/15 | 10.8.0.1 | 10.9.255.254 | 10.9.255.255 | 131,070 |
| 6 | 10.10.0.0/15 | 10.10.0.1 | 10.11.255.254 | 10.11.255.255 | 131,070 |
| 7 | 10.12.0.0/15 | 10.12.0.1 | 10.13.255.254 | 10.13.255.255 | 131,070 |
| 8 | 10.14.0.0/15 | 10.14.0.1 | 10.15.255.254 | 10.15.255.255 | 131,070 |
| 9 | 10.16.0.0/15 | 10.16.0.1 | 10.17.255.254 | 10.17.255.255 | 131,070 |
| 10 | 10.18.0.0/15 | 10.18.0.1 | 10.19.255.254 | 10.19.255.255 | 131,070 |
| 11 | 10.20.0.0/15 | 10.20.0.1 | 10.21.255.254 | 10.21.255.255 | 131,070 |
| 12 | 10.22.0.0/15 | 10.22.0.1 | 10.23.255.254 | 10.23.255.255 | 131,070 |
| 13 | 10.24.0.0/15 | 10.24.0.1 | 10.25.255.254 | 10.25.255.255 | 131,070 |
| 14 | 10.26.0.0/15 | 10.26.0.1 | 10.27.255.254 | 10.27.255.255 | 131,070 |
| 15 | 10.28.0.0/15 | 10.28.0.1 | 10.29.255.254 | 10.29.255.255 | 131,070 |
| 16 | 10.30.0.0/15 | 10.30.0.1 | 10.31.255.254 | 10.31.255.255 | 131,070 |
| 17 | 10.32.0.0/15 | 10.32.0.1 | 10.33.255.254 | 10.33.255.255 | 131,070 |
| 18 | 10.34.0.0/15 | 10.34.0.1 | 10.35.255.254 | 10.35.255.255 | 131,070 |
| 19 | 10.36.0.0/15 | 10.36.0.1 | 10.37.255.254 | 10.37.255.255 | 131,070 |
| 20 | 10.38.0.0/15 | 10.38.0.1 | 10.39.255.254 | 10.39.255.255 | 131,070 |
| 21 | 10.40.0.0/15 | 10.40.0.1 | 10.41.255.254 | 10.41.255.255 | 131,070 |
| 22 | 10.42.0.0/15 | 10.42.0.1 | 10.43.255.254 | 10.43.255.255 | 131,070 |
| 23 | 10.44.0.0/15 | 10.44.0.1 | 10.45.255.254 | 10.45.255.255 | 131,070 |
| 24 | 10.46.0.0/15 | 10.46.0.1 | 10.47.255.254 | 10.47.255.255 | 131,070 |
| 25 | 10.48.0.0/15 | 10.48.0.1 | 10.49.255.254 | 10.49.255.255 | 131,070 |
| 26 | 10.50.0.0/15 | 10.50.0.1 | 10.51.255.254 | 10.51.255.255 | 131,070 |
| 27 | 10.52.0.0/15 | 10.52.0.1 | 10.53.255.254 | 10.53.255.255 | 131,070 |
| 28 | 10.54.0.0/15 | 10.54.0.1 | 10.55.255.254 | 10.55.255.255 | 131,070 |
| 29 | 10.56.0.0/15 | 10.56.0.1 | 10.57.255.254 | 10.57.255.255 | 131,070 |
| 30 | 10.58.0.0/15 | 10.58.0.1 | 10.59.255.254 | 10.59.255.255 | 131,070 |
| 31 | 10.60.0.0/15 | 10.60.0.1 | 10.61.255.254 | 10.61.255.255 | 131,070 |
| 32 | 10.62.0.0/15 | 10.62.0.1 | 10.63.255.254 | 10.63.255.255 | 131,070 |
| 33 | 10.64.0.0/15 | 10.64.0.1 | 10.65.255.254 | 10.65.255.255 | 131,070 |
| 34 | 10.66.0.0/15 | 10.66.0.1 | 10.67.255.254 | 10.67.255.255 | 131,070 |
| 35 | 10.68.0.0/15 | 10.68.0.1 | 10.69.255.254 | 10.69.255.255 | 131,070 |
| 36 | 10.70.0.0/15 | 10.70.0.1 | 10.71.255.254 | 10.71.255.255 | 131,070 |
| 37 | 10.72.0.0/15 | 10.72.0.1 | 10.73.255.254 | 10.73.255.255 | 131,070 |
| 38 | 10.74.0.0/15 | 10.74.0.1 | 10.75.255.254 | 10.75.255.255 | 131,070 |
| 39 | 10.76.0.0/15 | 10.76.0.1 | 10.77.255.254 | 10.77.255.255 | 131,070 |
| 40 | 10.78.0.0/15 | 10.78.0.1 | 10.79.255.254 | 10.79.255.255 | 131,070 |
| 41 | 10.80.0.0/15 | 10.80.0.1 | 10.81.255.254 | 10.81.255.255 | 131,070 |
| 42 | 10.82.0.0/15 | 10.82.0.1 | 10.83.255.254 | 10.83.255.255 | 131,070 |
| 43 | 10.84.0.0/15 | 10.84.0.1 | 10.85.255.254 | 10.85.255.255 | 131,070 |
| 44 | 10.86.0.0/15 | 10.86.0.1 | 10.87.255.254 | 10.87.255.255 | 131,070 |
| 45 | 10.88.0.0/15 | 10.88.0.1 | 10.89.255.254 | 10.89.255.255 | 131,070 |
| 46 | 10.90.0.0/15 | 10.90.0.1 | 10.91.255.254 | 10.91.255.255 | 131,070 |
| 47 | 10.92.0.0/15 | 10.92.0.1 | 10.93.255.254 | 10.93.255.255 | 131,070 |
| 48 | 10.94.0.0/15 | 10.94.0.1 | 10.95.255.254 | 10.95.255.255 | 131,070 |
| 49 | 10.96.0.0/15 | 10.96.0.1 | 10.97.255.254 | 10.97.255.255 | 131,070 |
| 50 | 10.98.0.0/15 | 10.98.0.1 | 10.99.255.254 | 10.99.255.255 | 131,070 |
| 51 | 10.100.0.0/15 | 10.100.0.1 | 10.101.255.254 | 10.101.255.255 | 131,070 |
| 52 | 10.102.0.0/15 | 10.102.0.1 | 10.103.255.254 | 10.103.255.255 | 131,070 |
| 53 | 10.104.0.0/15 | 10.104.0.1 | 10.105.255.254 | 10.105.255.255 | 131,070 |
| 54 | 10.106.0.0/15 | 10.106.0.1 | 10.107.255.254 | 10.107.255.255 | 131,070 |
| 55 | 10.108.0.0/15 | 10.108.0.1 | 10.109.255.254 | 10.109.255.255 | 131,070 |
| 56 | 10.110.0.0/15 | 10.110.0.1 | 10.111.255.254 | 10.111.255.255 | 131,070 |
| 57 | 10.112.0.0/15 | 10.112.0.1 | 10.113.255.254 | 10.113.255.255 | 131,070 |
| 58 | 10.114.0.0/15 | 10.114.0.1 | 10.115.255.254 | 10.115.255.255 | 131,070 |
| 59 | 10.116.0.0/15 | 10.116.0.1 | 10.117.255.254 | 10.117.255.255 | 131,070 |
| 60 | 10.118.0.0/15 | 10.118.0.1 | 10.119.255.254 | 10.119.255.255 | 131,070 |
| 61 | 10.120.0.0/15 | 10.120.0.1 | 10.121.255.254 | 10.121.255.255 | 131,070 |
| 62 | 10.122.0.0/15 | 10.122.0.1 | 10.123.255.254 | 10.123.255.255 | 131,070 |
| 63 | 10.124.0.0/15 | 10.124.0.1 | 10.125.255.254 | 10.125.255.255 | 131,070 |
| 64 | 10.126.0.0/15 | 10.126.0.1 | 10.127.255.254 | 10.127.255.255 | 131,070 |
只列出前 64 个，共 128 个子网。规律不变：下一个网络地址等于上一个加块大小。
练一下
给一个地址，前缀都是 /15。先自己算，再展开答案。
第 1 题：10.144.7.75/15 所在的网络、可用范围和广播地址
- 网络地址
10.144.0.0/15- 可用范围
10.144.0.1到10.145.255.254- 广播地址
10.145.255.255- 可用主机
- 131,070
第 2 题：10.45.42.103/15 所在的网络、可用范围和广播地址
- 网络地址
10.44.0.0/15- 可用范围
10.44.0.1到10.45.255.254- 广播地址
10.45.255.255- 可用主机
- 131,070
第 3 题：10.10.203.150/15 所在的网络、可用范围和广播地址
- 网络地址
10.10.0.0/15- 可用范围
10.10.0.1到10.11.255.254- 广播地址
10.11.255.255- 可用主机
- 131,070
第 4 题：10.142.52.40/15 所在的网络、可用范围和广播地址
- 网络地址
10.142.0.0/15- 可用范围
10.142.0.1到10.143.255.254- 广播地址
10.143.255.255- 可用主机
- 131,070
第 5 题：10.51.252.13/15 所在的网络、可用范围和广播地址
- 网络地址
10.50.0.0/15- 可用范围
10.50.0.1到10.51.255.254- 广播地址
10.51.255.255- 可用主机
- 131,070
想多练：按前缀分组的练习题。