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Creating Subnets

Extending the Use of Address Masking

The original conception for three default masks defined by the leading bits in the address field was extended to allow installers and administrators to specify any mask value they wanted to use. In this way the restrictions placed on the addresses by the original masking were removed. The requirement remained, however, that the original masking be honored. It could be extended but not shortened. So, suppose that a site was assigned the address 140.6.0.0. This would imply that all stations at the site were on the same network. The default masking forced the use of the first two octets to identify the network and the remaining octets identify the hosts. If this site wanted to subdivide into separate networks, however, there was no facility in the original scheme to allow the address to reflect anything other than one level of hierarchical division; the world is divided into networks - end of story.

Because the mask was now a configurable parameter, the Class B network 140.6.0.0 could use a mask of 255.255.255.0, like a Class C network. The rest of the universe would see all stations at this site as being part of network 140.6.0.0 but within the site the routers would see the world as being divided on the basis of the first three octets. These are referred to as subnetworks. The site would be divided into subnetworks 140.6.1.0, 140.6.2.0, up to 140.6.255.0.

The extension of the original default address masks to allow any desired mask parameter assumes compliance with the original class masks. That is, a Class A network must have a mask of at least 255.0.0.0, a Class B network must have a mask of at least 255.255.0.0, and a Class C network must have at least 255.255.255.0. A Class B network could, for example, have a mask of 255.255.255.0 or any other mask as long as the first two octets are included in the mask; to remain compliant with the original mask definitions.

The extension of the address mask allows networks to be further subdivided into sub networks. For example, the Class B network 140.6.0.0 could be subnetted with 255.255.255.0 as a mask. This would create subnets like 140.6.1.0, 140.6.2.0,... up to 140.6.255.0.