IPv4 Subnet Masks, CIDR Notation, and Host Counts
Subnet Masks
| Class | Subnet Mask | Binary Subnet Mask |
|---|---|---|
| A | 255.0.0.0 | 11111111.00000000.00000000.00000000 |
| B | 255.255.0.0 | 11111111.11111111.00000000.00000000 |
| C | 255.255.255.0 | 11111111.11111111.11111111.00000000 |
| Ones (1's) are network bits. Zeros (0's) are node bits (assignable address bits). | ||
IPv4 Address Counts by Class
| Class | Total Addresses |
|---|---|
| A | 16,777,216 (2^24) |
| B | 65,536 (2^16) |
| C | 256 (2^8) |
These are total addresses in each class range; usable host counts exclude the network and broadcast addresses.
CIDR Subnet Mask and Usable Host Reference
| Bits | Subnet Mask | Subnets Within a /8 Network | Usable Hosts |
|---|---|---|---|
| 8 | 255.0.0.0 | 1 | 16,777,214 |
| 16 | 255.255.0.0 | 256 | 65,534 |
| 24 | 255.255.255.0 | 65,536 | 254 |
| 25 | 255.255.255.128 | 131,072 | 126 |
| 26 | 255.255.255.192 | 262,144 | 62 |
| 27 | 255.255.255.224 | 524,288 | 30 |
| 28 | 255.255.255.240 | 1,048,576 | 14 |
| 29 | 255.255.255.248 | 2,097,152 | 6 |
| 30 | 255.255.255.252 | 4,194,304 | 2 |
Subnet counts use a class A /8 network as the reference. Usable host counts exclude the network and broadcast addresses.
Binary Subnet Representation
| Decimal | Binary | Bits |
|---|---|---|
| .128 | 10000000 | 1 |
| .192 | 11000000 | 2 |
| .224 | 11100000 | 3 |
| .240 | 11110000 | 4 |
| .248 | 11111000 | 5 |
| .252 | 11111100 | 6 |
| .254 | 11111110 | 7 |
| .255 | 11111111 | 8 |