For private networks a certain number of IP addresses are allocated which are never used on the Internet. These reserved IPs are typically used for LANs.
The following table displays the various private/reserved classes.
Class A: 8-bit network address and 24-bit host address
The first byte of the IP number is reserved for the network address. So the default subnet mask would be 255.0.0.0. The 3 remaining bytes are available to set host interfaces.
Since 255.255.255 and 0.0.0 are invalid host numbers there are 224 - 2 = 16777214 possible hosts.
IP numbers have the first byte ranging from 1 to 127. This corresponds to a binary range of 00000001 to 01111111. The first bit of a class A address is always set to 0.
Class B: 16-bit network address and 16-bit host address
The two first bytes of the IP number are reserved for the network address. The default subnet mask is 255.255.0.0. There are 216 2 = 65534 possible hosts.
The first byte ranges from 128 to 191. Notice that the binary range of the first byte is 10000000 to 10111111. That is the first two bits of a class B address are always set to 10.
Class C: 24-bit network address and 8-bit host address
The three first bytes are reserved for the network address. The default subnet mask is 255.255.255.0. There are 28 - 2 = 254 possible hosts.
The first byte ranges from 192 to 223. This corresponds to a binary range from 11000000 to 11011111. From this we conclude that the first three bits of a class C address are always set to 110.