Lucent Technologies Release 8.2 manual Subnetting, How subnets are created

IP Addressing 1 Networking Overview

Subnetting

Subnetting is the grouping of IP addresses associated with a network ID into two or more subnetworks. The subnets of a network ID are visible only within the organization that owns the network ID; Internet routers route messages based on the network ID and the routers within the private organization differentiate between the individual subnets.

Reasons for subnetting Subnetting is desirable because it enables a more efficient allocation and management of IP addresses.

The three-class hierarchy of IP addresses results in an inefficient allocation of addresses in many cases because addresses are assigned and managed in blocks by network ID. For example, a company that needs 10,000 IP addresses in each of two locations might be assigned two Class B network IDs, each of which provides 65,534 IP addresses. Even though one Class B network ID would provide more than enough addresses for both locations, having a separate network ID for each location is easier to manage. If the company uses only 20,000 of these addresses, about 100,000 go unused.

In this case, subnetting would enable the company to use one Class B network ID and subdivide the addresses into two subnets, one for each location. Each subnet would have a unique “extended network ID” that would enable them to be managed as if they had unique network IDs.

Typically, organizations need to manage IP addresses in separate groups based on several criteria in addition to location:

•different types of LANs

•different server applications

•different work projects

•security

How subnets are created

The grouping of IP addresses provided by the three-Class structure does not allow nearly enough flexibility to meet the needs of most organizations. Subnetting allows the N IP addresses associated with a network ID to be divided into as few as 2 groups, each with N/2 addresses, or into as many as N/2 groups, each with 2 addresses, if desired.

RFC 950 defines a standard procedure to divide a Class A, B, or C network ID into subnets. The subnetting adds a third level of hierarchy to the two-level hierarchy of the Class A, B, and C network ID number. An “extended network prefix” is formed by using two or more bits of the Host ID as a subnet number, and appending this subnet number to the network ID.

		Administration for Network Connectivity
16	CID: 77730	555-233-504 — Issue 1 — April 2000