IP Addressing


The network layer, (Layer 3) of the OSI Model defines each device on the network. In IPv4, each packet uses a 32-bit source address and a 32-bit destination address in the Layer 3 header. These addresses are represented in a dotted decimal format.

IPv4 addresses are easier to remember while in the dotted decimal format rather than in their natural binary state. For example, take the address This is a lot easier to remember than: 10101100000100000000010000010100 is it not?

Network and Hosts

There are two parts to IPv4 address, the Network and the Host portions. The network portion of an IPv4 address is known as the high-order bits because they are the most significant. A group of hosts that have identical bit patterns is known as the network. For example, take these two addresses: and The portion of the IP address in bold represents the network portion of the address, the portion in black is the host part of the address.

IPv4 Network Classes

There are five IPv4 Network classes, (A, B, C, D & E,) but only the three most used, (A, B & C) will be focused on. Class D is a Multicast address while Class E is used for experimental and future use. Here are Classes A, B & C's basic information. A Class A, B or C address space is known as Classful Addressing.

Classful & Classless Addressing

Classful Addressing

This addressing method divides the address space for Internet Protocol Version 4 (IPv4) into five address classes. Each class, coded in the first four bits of the address, defines either a different network size. In other words, each size of the network is defined and has a "Class" associated with it.

Classless Addressing

Classless addressing uses a variable number of bits for the network and host portions of the address. They are not a fixed size and do not have a particular "Class" associated with it.

Class 1st Octet Range Prefix & Mask # of Networks # of Hosts
A 1 to 127 /8 126 (27) 16,777,214 (224 -2)
B 128 to 191 /16 16,384 (214) 65,534 (216 -2)
C 192 to 223 /24 2,097,159(221) 254 (28 -2)

IPv4 Class Address Blocks

Class A Blocks

The Class A Block was designed to support large networks with more than 16 million host addresses.

Class B Blocks

The Class B Block was designed to support moderate to large networks with more than 65,000 host addresses.

Class C Blocks

The Class C Block was designed to support small networks with a maximum of 254 host addresses.