Classes of network IP addresses

Classes of network IP addresses
10.3.1 This page will review the classes of IP addresses. The combined classes of IP addresses offer a range from 256 to 16.8 million hosts.


To efficiently manage a limited supply of IP addresses, all classes can be subdivided into smaller subnetworks. Figure provides an overview of the division between networks and hosts.

The next page will explain why subnetting is important

Introduction to and reason for subnetting
10.3.2 This page will describe how subnetting works and why it is important.


To create the subnetwork structure, host bits must be reassigned as network bits. This is often referred to as ‘borrowing’ bits. However, a more accurate term would be ‘lending’ bits. The starting point for this process is always the leftmost host bit, the one closest to the last network octet.

Subnet addresses include the Class A, Class B, and Class C network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion of the major IP address. This is done by re-assigning bits from the host portion to the original network portion of the address. - The ability to divide the original host portion of the address into the new subnet and host fields provides addressing flexibility for the network administrator.

In addition to the need for manageability, subnetting enables the network administrator to provide broadcast containment and low-level security on the LAN. Subnetting provides some security since access to other subnets is only available through the services of a router. Further, access security may be provided through the use of access lists. These lists can permit or deny access to a subnet, based on a variety of criteria, thereby providing more security. Access lists will be studied later in the curriculum. Some owners of Class A and B networks have also discovered that subnetting creates a revenue source for the organization through the leasing or sale of previously unused IP addresses.

Subnetting is an internal function of a network. From the outside, a LAN is seen as a single network with no details of the internal network structure. This view of the network keeps the routing tables small and efficient. Given a local node address of 147.10.43.14 on subnet 147.10.43.0, the world outside the LAN sees only the advertised major network number of 147.10.0.0. The reason for this is that the local subnet address of 147.10.43.0 is only valid within the LAN where subnetting is applied.

The next page will discuss subnet masks.