| Start | End | No. of addresses | |
|---|---|---|---|
| 24-bit block (/8 prefix, 1 × A) | 10.0.0.0 | 10.255.255.255 | 16777216 |
| 20-bit block (/12 prefix, 16 × B) | 172.16.0.0 | 172.31.255.255 | 1048576 |
| 16-bit block (/16 prefix, 256 × C) | 192.168.0.0 | 192.168.255.255 | 65536 |
| Class | Leading bits |
Size of network number bit field |
Size of rest bit field |
Number of networks |
Addresses per network |
Start address | End address |
|---|---|---|---|---|---|---|---|
| A | 0 | 8 | 24 | 128 (27) | 16,777,216 (224) | 0.0.0.0 | 127.255.255.255 |
| B | 10 | 16 | 16 | 16,384 (214) | 65,536 (216) | 128.0.0.0 | 191.255.255.255 |
| C | 110 | 24 | 8 | 2,097,152 (221) | 256 (28) | 192.0.0.0 | 223.255.255.255 |
Name resolution
IPv4 network standards reserve the entire 127.0.0.0/8 address block for loopback purposes. That means any packet sent to one of those 16,777,214 addresses (127.0.0.1 through 127.255.255.254) will be looped back.
The resolution of the name ‘localhost’ to the most-commonly used loopback addresses is set up by the following lines in each device’s hosts file:
127.0.0.1 localhost ::1 localhost
TECH INFO 