先决条件——无类寻址、超网
问题 –如何计算 IP 地址子网信息(网络、广播、第一个 IP、最后一个 IP)?
计算 IPv4 子网网络 ID 的公式太简单了。
*在 C 类子网中使用和测试。
首先，记住这个子网主机映射(每个前缀的主机数)：
Network Prefix: Number of IPs
24 : 256 IPs
25 : 128 IPs
26 : 64 IPs
27 : 32 IPs
28 : 16 IPs
29 : 8 IPs
30 : 4 IPs 使用方程：
Network ID: floor(Host Address/Subnet Number of Hosts) * Subnet Number of Hosts
Broadcast ID: (Host ID + (Subnet Number of Hosts-1))
First Host: Network ID + 1
Last Host: Broadcast ID - 1
例 1：192.168.1.65/28：
65/16 = 4.0625
Network ID: 4*16 = 64 (192.168.1.64)
Broadcast ID: 64+(16-1) = 79 (192.168.1.79)
First Host ID: 64 + 1 = 65 (192.168.1.65)
Last Host ID: 79 - 1 = 78 (192.168.1.78)
例2：192.168.20.166/25：
166/128 = 1.296875
Network ID: 1*128 = 128 (192.168.20.128)
Broadcast ID: 128+(128-1) = 255 (192.168.20.255)
First Host ID: 128 + 1 = 129 (192.168.20.129)
Last Host ID: 255 - 1 = 254 (192.168.20.254)
例3：192.168.30.14/29：
14/8 = 1.75
Network ID: 1*8 = 8 (192.168.30.8)
Broadcast ID: 8+(8-1) = 15 (192.168.30.15)
First Host ID: 8 + 1 = 9 (192.168.30.9)
Last Host ID: 15 - 1 = 14 (192.168.30.14)
例 4：192.168.20.86/30：
86/4 = 21.5
Network ID: 21*4 = 84 (192.168.20.84)
Broadcast ID: 84+(4-1) = 87 (192.168.20.87)
First Host ID: 84 + 1 = 85 (192.168.20.85)
Last Host ID: 87 - 1 = 86 (192.168.20.86)