无类别域间路由
前缀聚合

CIDR的另一个好处就是可以进行前缀路由聚合。例如，16个原来的C类（/24）网络现在可以聚合在一起，对外显示了一个/20的网络了（如果这些网络的的地址前20位都相同）。两个对齐的/20网络又可进一步聚合为/19，依此类推。这有效地减少了要对外显示的网络数，防止了路由表爆炸，也遏制了互联网进一步扩大。
| IP/CIDR | Δ 与最后一个地址的差值 | 掩码 | 主机数 (*) | 类别 | 备注 |
| --- | --- | --- | --- | --- | --- |
| a.b.c.d/32 | +0.0.0.0 | 255.255.255.255 | 1 | 1/256 C |  |
| a.b.c.d/31 | +0.0.0.1 | 255.255.255.254 | 2 | 1/128 C | d = 0 ... (2n) ... 254 |
| a.b.c.d/30 | +0.0.0.3 | 255.255.255.252 | 4 | 1/64 C | d = 0 ... (4n) ... 252 |
| a.b.c.d/29 | +0.0.0.7 | 255.255.255.248 | 8 | 1/32 C | d = 0 ... (8n) ... 248 |
| a.b.c.d/28 | +0.0.0.15 | 255.255.255.240 | 16 | 1/16 C | d = 0 ... (16n) ... 240 |
| a.b.c.d/27 | +0.0.0.31 | 255.255.255.224 | 32 | 1/8 C | d = 0 ... (32n) ... 224 |
| a.b.c.d/26 | +0.0.0.63 | 255.255.255.192 | 64 | 1/4 C | d = 0, 64, 128, 192 |
| a.b.c.d/25 | +0.0.0.127 | 255.255.255.128 | 128 | 1/2 C | d = 0, 128 |
| a.b.c.0/24 | +0.0.0.255 | 255.255.255.000 | 256 | 1 C |  |
| a.b.c.0/23 | +0.0.1.255 | 255.255.254.000 | 512 | 2 C | c = 0 ... (2n) ... 254 |
| a.b.c.0/22 | +0.0.3.255 | 255.255.252.000 | 1,024 | 4 C | c = 0 ... (4n) ... 252 |
| a.b.c.0/21 | +0.0.7.255 | 255.255.248.000 | 2,048 | 8 C | c = 0 ... (8n) ... 248 |
| a.b.c.0/20 | +0.0.15.255 | 255.255.240.000 | 4,096 | 16 C | c = 0 ... (16n) ... 240 |
| a.b.c.0/19 | +0.0.31.255 | 255.255.224.000 | 8,192 | 32 C | c = 0 ... (32n) ... 224 |
| a.b.c.0/18 | +0.0.63.255 | 255.255.192.000 | 16,384 | 64 C | c = 0, 64, 128, 192 |
| a.b.c.0/17 | +0.0.127.255 | 255.255.128.000 | 32,768 | 128 C | c = 0, 128 |
| a.b.0.0/16 | +0.0.255.255 | 255.255.000.000 | 65,536 | 256 C = 1 B |  |
| a.b.0.0/15 | +0.1.255.255 | 255.254.000.000 | 131,072 | 2 B | b = 0 ... (2n) ... 254 |
| a.b.0.0/14 | +0.3.255.255 | 255.252.000.000 | 262,144 | 4 B | b = 0 ... (4n) ... 252 |
| a.b.0.0/13 | +0.7.255.255 | 255.248.000.000 | 524,288 | 8 B | b = 0 ... (8n) ... 248 |
| a.b.0.0/12 | +0.15.255.255 | 255.240.000.000 | 1,048,576 | 16 B | b = 0 ... (16n) ... 240 |
| a.b.0.0/11 | +0.31.255.255 | 255.224.000.000 | 2,097,152 | 32 B | b = 0 ... (32n) ... 224 |
| a.b.0.0/10 | +0.63.255.255 | 255.192.000.000 | 4,194,304 | 64 B | b = 0, 64, 128, 192 |
| a.b.0.0/9 | +0.127.255.255 | 255.128.000.000 | 8,388,608 | 128 B | b = 0, 128 |
| a.0.0.0/8 | +0.255.255.255 | 255.000.000.000 | 16,777,216 | 256 B = 1 A |  |
| a.0.0.0/7 | +1.255.255.255 | 254.000.000.000 | 33,554,432 | 2 A | a = 0 ... (2n) ... 254 |
| a.0.0.0/6 | +3.255.255.255 | 252.000.000.000 | 67,108,864 | 4 A | a = 0 ... (4n) ... 252 |
| a.0.0.0/5 | +7.255.255.255 | 248.000.000.000 | 134,217,728 | 8 A | a = 0 ... (8n) ... 248 |
| a.0.0.0/4 | +15.255.255.255 | 240.000.000.000 | 268,435,456 | 16 A | a = 0 ... (16n) ... 240 |
| a.0.0.0/3 | +31.255.255.255 | 224.000.000.000 | 536,870,912 | 32 A | a = 0 ... (32n) ... 224 |
| a.0.0.0/2 | +63.255.255.255 | 192.000.000.000 | 1,073,741,824 | 64 A | a = 0, 64, 128, 192 |
| a.0.0.0/1 | +127.255.255.255 | 128.000.000.000 | 2,147,483,648 | 128 A | a = 0, 128 |
| 0.0.0.0/0 | +255.255.255.255 | 000.000.000.000 | 4,294,967,296 | 256 A |  |
通常来说，子网中的第一个地址（主机标识符中的所有二进制零的地址）都保留用于引用网络本身，而最后一个地址用作广播地址用于网络；这样可以将可用于主机的地址数量减少2个。结果，主机标识符中只有一个二进制数字的/31网络将无法使用，因为这样的子网在减少之后将不提供可用的主机地址。RFC 3021为“全为一的主机”和“全为零的主机”规则创建了一个例外，以使/31网络可用于点对点链接。/31地址（单主机网络）必须通过明确的路由规则访问，因为在这种网络中没有网关的空间。
在大于/31或/32的路由子网中，可用主机地址的数目通常减少两个，即保留作为广播地址的最大地址和标识网络本身的最小地址。