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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP "Classful" (Conventional) Addressing

IP "Classful" Addressing Overview and Address Classes 1 2 3 IP Address Class A, B and C Network and Host Capacities

Determining Address Class From the First Octet Bit Pattern

As humans, of course, we generally work with addresses in dotted decimal notation and not in binary, but it's pretty easy to see the ranges that correspond to the classes. For example, consider class B. The first two bits of the first octet are “10”. The remaining bits can be any combination of ones and zeroes. This is normally represented as “10xx xxxx” (shown as two groups of four for readability.) 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 (inclusive) is a class B address.

In Table 44 I have shown the bit patterns of each of the five classes, and the way that the first octet ranges can be calculated. In the first column is the format for the first octet of the IP address, where the “x”s can be either a zero or a one. Then I show the lowest and highest value for each class in binary (the “fixed” few bits are highlighted so you can see that they do not change while the others do.) I then also show the corresponding range for the first octet in decimal.

IP "Classful" Addressing Overview and Address Classes 1 2 3 IP Address Class A, B and C Network and Host Capacities