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Network Address Range: Class A

The designers of the IP address scheme said that the first bit of the first byte in a Class A network address must always be off, or 0. This means a Class A address must be between 0 and 127 in the first byte, inclusive.

Consider the following network address:

0xxxxxxx

If we turn the other 7 bits all off and then turn them all on, we’ll find the Class A range of network addresses:

00000000 = 0
01111111 = 127

So, a Class A network is defined in the first octet between 0 and 127, and it can’t be less or more. (Yes, I know 0 and 127 are not valid in a Class A network. I’ll talk about reserved addresses in a minute.)
Network Address Range: Class B

In a Class B network, the RFCs state that the first bit of the first byte must always be turned on but the second bit must always be turned off. If you turn the other 6 bits all off and then
all on, you will find the range for a Class B network:

10000000 = 128
10111111 = 191

As you can see, a Class B network is defined when the first byte is configured from 128 to 191.
Network Address Range: Class C

For Class C networks, the RFCs define the first 2 bits of the first octet as always turned on, but the third bit can never be on. Following the same process as the previous classes, convert from binary to decimal to find the range. Here’s the range for a Class C network:

11000000 = 192
11011111 = 223

So, if you see an IP address that starts at 192 and goes to 223, you’ll know it is a Class C IP address.
Network Address Ranges: Classes D and E

The addresses between 224 to 255 are reserved for Class D and E networks. Class D (224–239) is used for multicast addresses and Class E (240–255) for scientific purposes, but I’m not going into these types of addresses in this book (and you don’t need to know them).

Remember the Class A range. The IP range for a Class A network is 1–126. This provides 8 bits of network addressing and 24 bits of host addressing by default.
Remember the Class B range. The IP range for a Class B network is 128–191. Class B
addressing provides 16 bits of network addressing and 16 bits of host addressing by default.
Remember the Class C range. The IP range for a Class C network is 192–223. Class C
addressing provides 24 bits of network addressing and 8 bits of host addressing by default.
Remember the Private IP ranges.

Class A private address range is 10.0.0.0 through 10.255.255.255.
Class B private address range is 172.16.0.0 through 172.31.255.255.
Class C private address range is 192.168.0.0 through 192.168.255.255.