It is also for establishing the host address size for any given IP address. The address mask, which is so commonly associated with an IP address today, was not required as the mask length was purely a function of the IP address. Any network device could inspect the first few bits of the 32 bit IP address to see which class it belonged to.
| Class |
start |
end |
first bits |
Mask (in dotted decimal) |
| Class A |
1.0.0.0 |
126.0.0.0 |
0 |
255.0.0.0 |
| Class B |
128.0.0.0 |
191.255.0.0 |
10 |
255.255.0.0 |
| Class C |
192.0.0.0 |
223.255.255.0 |
110 |
255.255.255.0 |
| Class D (multicast) |
224.0.0.0 |
239.255.255.255 |
1111 |
n/a |
| Class F (experimental) |
n/a |
The 127.0.0.0 network is reserved for loopback addresses.
Early allocations of IP addresses by IANA were in some cases not made very efficiently. This resulted with some organisations receiving class A networks which has contributed to the IP address shortage.