First Octet Bit for Address Class

When configuring TCP/IP, we generally work with network addresses in dotted decimal and not in binary, but it's pretty easy to see the ranges that correspond to the class address or class addresses. For example, using class B. The first two bits of the first octet are “10”. The remaining bits can be change to one, for us to determine the its range. This is represented as “10xx xxxx”. Thus, the binary range for the first octet can be from “1000 0000” to “1011 1111”. This is 128 to 191 in decimal. So, in the “classful” scheme, any IP address whose first octet is from 128 to 191 is a class B address.


Below example, I have shown the bit patterns of each of the five classes, and the way that the first octet ranges can be calculated.


IP Address Class

Class A
0xxx xxxx - total 1
0111 1111 - total 127

range: 1 to 127

Class B
10xx xxxx
1000 0000 - total 128
1011 1111 - total 191

range: 128 to 191

Class C
110x xxxx
1100 0000 - total 192
1101 1111 - total 223

range: 192 to 223

Class D
1110 xxxx
1110 0000 - total 224
1110 1111 - total 239

range: 224 to 239

Class E
1111 0000 - total 240
range: 240 to 254


note: Class D and Class E are reserved and not usable. It was meant for multicasting and research.

Also take notes that most of the ip address above are public ip addresses. To determine the private ip address are the following:

Class A

range: 10.0.0.0 to 10.255.255.255

Class B

range: 172.16.0.0 to 172.31.255.255

Class C

range: 192.168.0.0 to 192.168.255.255

These private ip address is what we used on our LAN network.