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10.3 | |
The Mechanics of Subnetting | |
| 10.3.2 | |
Introduction to and reason for subnetting |
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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
assigning bits from the host portion to the original network portion
of the address.
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. A LAN is seen as a single network with no knowledge of the internal network structure. This view of the network keeps the routing tables small and efficient. Given a local node address of 192.168.10.14, the world outside the LAN sees only the advertised major network number of 192.168.10.0. The reason for this is that the local address of 192.168.10.14 is only valid within the LAN 192.168.10.0 and cannot function anywhere else.
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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. 
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