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Purpose of Subnet Masks:

This page explains the purpose of subnetting and subnet masks within the context of internet Technologies
A viable solution to creating new network addresses is to divide the 32-bit address in a process called subnetting   Internet Web Technologies
Lesson 5Subnets
ObjectiveExplain the purpose of subnetting and subnet masks.
What is subnetting?
There is a scarcity of network addresses today. A viable solution to creating new network addresses is to divide the 32-bit address in a process called subnetting.
The process entails modifying the IP address by using the host (or client portion) address bits as network address bits. A more technical evaluation is that a subnet mask is applied to filter the bits not used in the network address.
A subnet mask is a 32-bit number, written in dotted decimal form, in much the same way an IP address is displayed. For example
only hosts IP addresses that are on the same subnet and can communicate with others without a router.
Subnetting is used when a company has more computers than can be connected to its network; it is also used as a traffic reduction mechanism. However, all devices on the subnet must be programmed with the correct mask value in order for the network to function properly.
Since subnetting tricks the IP Address by turning an unused portion of the Public Address into a Privated address (as discussed at the end of Lesson 4). Subnetting can only occur within a LAN setting, and required a great deal of caution. An incorrect subnetting scheme may violate IP addressing standards, which could cause the network address to be misinterpreted by Internet protocols. This would result in widespread communications problems.

Network Fundamentals
Subnet masks
To subnet an IP network, you use a subnet mask, which splits IP networks into subgroups of networks or subnetworks by filtering unwanted bits so that only usable bits remain. Before using subnet masks, you must perform a decimal-to-binary conversion of the IP address. A subnet mask identifies how the bits of an IP address are used. In binary terms, 0 and 1 are the only functional digits. In binary notation, a 0 (zero) indicates that the bit is a host bit. A 1 (one) indicates that the bit is a network bit.
A subnet mask defines the network, the subnet, and the host.
The table below provides the default masks for IP address classes A to C.
Class Default mask
In the next lesson, you will learn about remote access protocols.