Brought to you by EarthWeb
IT Library Logo

Click Here!
Click Here!


Search the site:
 
EXPERT SEARCH -----
Programming Languages
Databases
Security
Web Services
Network Services
Middleware
Components
Operating Systems
User Interfaces
Groupware & Collaboration
Content Management
Productivity Applications
Hardware
Fun & Games

EarthWeb Direct EarthWeb Direct Fatbrain Auctions Support Source Answers

EarthWeb sites
Crossnodes
Datamation
Developer.com
DICE
EarthWeb.com
EarthWeb Direct
ERP Hub
Gamelan
GoCertify.com
HTMLGoodies
Intranet Journal
IT Knowledge
IT Library
JavaGoodies
JARS
JavaScripts.com
open source IT
RoadCoders
Y2K Info

Previous Table of Contents Next


Improved Address Space Utilization

In addition to improved scalability of the routing system, CIDR also improves address space utilization. At the level of individual data link subnetworks the effect of CIDR on address space utilization is very similar (if not identical) to the variable-length subnets. Because with CIDR there are no constraints on the length of the prefix, the number of the addresses covered by the prefix assigned to a given subnetwork could closely match the number of hosts on that subnetwork, thereby promoting efficient address space utilization at the level of individual subnetworks.

In contrast with pre-CIDR address allocation procedures, where the smallest unit of allocation to a subscriber was a single network number, with CIDR the amount of address space allocated to a subscriber is determined solely by the number of hosts within the subscriber. This allows address space utilization to be improved at the level of individual subscribers. Because CIDR has no notion of network number classes, CIDR allows efficient address allocation across the whole IP unicast address space, including the address space associated with the Class A network numbers. By improving address space utilization, CIDR makes it possible to significantly defer the potential exhaustion of the IP address space.

Supporting CIDR requires IP routing protocols to treat addressing information just as address prefixes, without any notion of network classes. Intradomain routing protocols that provide this functionality are OSPF, RIP-II, EIGRP, and Dual IS-IS. BGP-4 is an inter-domain routing protocol that provides this functionality.

CIDR also requires IP forwarding to support the longest match algorithm. With this algorithm, when a router has to forward a packet, the router matches the destination address of the packet against all the entries in the router’s forwarding table. The entry that provides the longest match is used for forwarding the packet.

Impact on Address Allocation and Management

An IP address, by itself, can be viewed as just a sequence of 32 bits, with no intrinsic value. An IP address acquires its value as the information about this address gets injected into the routing system. In the context of the Internet, an IP address acquires its value once the information about the address gets injected into the Internet routing system. Because it would be fairly pointless to have allocation and management policies for numbers with no intrinsic value, for any IP address allocation and management policy not to be self-destructive, such a policy must ensure that it would not result in the collapse of the routing system. To maintain operational Internet, address allocation in the Internet can no longer be done without considering its impact on the Internet routing system-Internet address allocation and management policies must promote scalable routing.

The need to scale the Internet routing system, and the use of CIDR as the primary tool to accomplish such scaling, caused the evolution of the Internet address allocation and management policies. Extending the notion of hierarchical routing to the level of individual subscribers and providers, and allowing subscribers and providers to act as aggregators of addressing information required changes to the address allocation procedures. Although in the pre-CIDR days address allocation to subscribers was done without taking into consideration the need to aggregate the addressing information above the level of individual subscribers, CIDR-based allocation requires that address allocation be done in such a way, as to enable subscribers and providers to act as aggregators of addressing information—such an allocation is called the aggregator-based address allocation.

In the pre-CIDR days, it was assumed that once a subscriber acquires a block of addresses, the subscriber could use these addresses to gain access to the Internet routing system (and thereby obtain Internet-wide connectivity). It was further assumed that the subscriber would be able to change its providers, while being able to keep the same addresses and maintain access to the Internet routing system. The address allocation policy that was based on these assumptions is called address ownership. The policy assumes that once an organization (i.e., a subscriber) is allocated a block of addresses, the organization can use these addresses for as long as it wishes for the purpose of obtaining access to the Internet routing system (and thus obtaining Internet-wide connectivity). An important implication of this assumption was that routers in the Internet (and specifically routers that do not rely on the default route) would be able to maintain routes to all the individual subscribers connected to the Internet.


Previous Table of Contents Next

footer nav
Use of this site is subject certain Terms & Conditions.
Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.