Cisco Netmasks, CIDR sizes and Inverse Masks
| Netmask | Inverse | /CIDR | Usable | Size |
| 0.0.0.0 | 255.255.255.255 | /0 | 4,294,967,294 | The Internet |
| 128.0.0.0 | 127.255.255.255 | /1 | 2,147,483,646 | 128 Class 'A's |
| 192.0.0.0 | 63.255.255.255 | /2 | 1,073,741,822 | 64 Class 'A's |
| 224.0.0.0 | 31.255.255.255 | /3 | 536,870,910 | 32 Class 'A's |
| 240.0.0.0 | 15.255.255.255 | /4 | 268,435,454 | 16 Class 'A's |
| 248.0.0.0 | 7.255.255.255 | /5 | 134,217,726 | 8 Class 'A's |
| 252.0.0.0 | 3.255.255.255 | /6 | 67,108,862 | 4 Class 'A's |
| 254.0.0.0 | 1.255.255.255 | /7 | 33,554,430 | 2 Class 'A's |
| 255.0.0.0 | 0.255.255.255 | /8 | 16,777,214 | 1 Class 'A' |
| 255.128.0.0 | 0.127.255.255 | /9 | 8,388,606 | 128 Class 'B's |
| 255.192.0.0 | 0.63.255.255 | /10 | 4,194,302 | 64 Class 'B's |
| 255.224.0.0 | 0.31.255.255 | /11 | 2,097,150 | 32 Class 'B's |
| 255.240.0.0 | 0.15.255.255 | /12 | 1,048,574 | 16 Class 'B's |
| 255.248.0.0 | 0.7.255.255 | /13 | 524,286 | 8 Class 'B's |
| 255.252.0.0 | 0.3.255.255 | /14 | 262,142 | 4 Class 'B's |
| 255.254.0.0 | 0.1.255.255 | /15 | 131,070 | 2 Class 'B's |
| 255.255.0.0 | 0.0.255.255 | /16 | 65,534 | 1 Class 'B' |
| 255.255.128.0 | 0.0.127.255 | /17 | 32,766 | 128 Class 'C's |
| 255.255.192.0 | 0.0.63.255 | /18 | 16,382 | 64 Class 'C's |
| 255.255.224.0 | 0.0.31.255 | /19 | 8,190 | 32 Class 'C's |
| 255.255.240.0 | 0.0.15.255 | /20 | 4,094 | 16 Class 'C's |
| 255.255.248.0 | 0.0.7.255 | /21 | 2,046 | 8 Class 'C's |
| 255.255.252.0 | 0.0.3.255 | /22 | 1,022 | 4 Class 'C's |
| 255.255.254.0 | 0.0.1.255 | /23 | 510 | 2 Class 'C's |
| 255.255.255.0 | 0.0.0.255 | /24 | 254 | 1 Class 'C' |
| 255.255.255.128 | 0.0.0.127 | /25 | 126 | 128 Hosts |
| 255.255.255.192 | 0.0.0.63 | /26 | 62 | 64 Hosts |
| 255.255.255.224 | 0.0.0.31 | /27 | 30 | 32 Hosts |
| 255.255.255.240 | 0.0.0.15 | /28 | 14 | 16 Hosts |
| 255.255.255.248 | 0.0.0.7 | /29 | 6 | 8 Hosts |
| 255.255.255.252 | 0.0.0.3 | /30 | 2 | 4 Hosts |
| 255.255.255.254 | 0.0.0.1 | /31 | 0 | 2 Hosts |
| 255.255.255.255 | 0.0.0.0 | /32 | 1 | 1 Host |
