Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity Basics Click Here! Search the site:   ITLibrary ITKnowledge 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 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 The need to improve scaling properties of the Internet routing system results in a situation where address allocations to subscribers based on the address ownership policy became highly detrimental to the Internet routing system. On one hand, such allocations effectively preclude aggregation of the addressing information above the level of individual subscribers, and force the Internet routing system to scale linearly with the number of subscribers. On the other hand, as mentioned earlier, the ability to aggregate addressing information at the level of providers (thereby making the routing system scale less than linearly with the number of subscribers) is essential to the continuous growth of the Internet. So, to support continuous growth of the Internet, it is necessary to evolve address allocation policies. If a subscriber does not provide a sufficient degree of addressing information aggregation, CIDR recommends that such a subscriber should use a subblock of addresses out of the block allocated to its provider. The provider then would act as the aggregator for the subscriber’s addressing information. If the subscriber changes its providers, the subscriber would no longer need the addresses it took out of the old provider. So the subscriber should return these addresses to the old provider. At the same time, the subscriber would need to get a subblock out of the block allocated to the new provider, so that the new provider would become the aggregator for the subscriber’s addressing information. In an environment where as a subscriber changes its providers the subscriber may change its addresses, the notion of owning addresses makes little or no sense for such a subscriber. Aggregator based address allocation suggests that in addition to the traditional address ownership policy, the address lending policy must be addressed. Address allocation based on this policy allows a subscriber to borrow its addresses out of its provider, while the provider acts as an aggregator for the subscriber. The address lending policy should be used for the address allocation to the organizations that by themselves do not provide a sufficient degree of routing information aggregation. Using the address lending policy enables such organizations to gain access to the Internet routing system, without overloading the system beyond its capacities. Impact on Renumbering Scaling capabilities of CIDR are based on the assumption that address allocation reflects network topology. This assumption is especially important at the level of subscribers, and their interconnection with providers, as to enable subscribers and providers to act as aggregators. If a subscriber changes its provider, then to avoid injecting additional overhead in the Internet routing system, subscriber may need to renumber. Although CIDR does not require every subscriber that changes its providers to renumber, it is important to stress that if none of the subscribers that change their providers will renumber, the Internet routing system would collapse as a result of the excessive amount of routing information it would need to handle. Maintaining aggregator-based address allocation (to promote scalable routing), and the need to support the ability of subscribers to change their providers (to promote competition) demands practical solutions for renumbering. The need to contain the routing overhead in a rapidly growing Internet with multiple competing providers is likely to make renumbering more and more common. Use of Dynamic Host Configuration Protocol (DHCP) is expected to significantly facilitate renumbering, as it would automate host renumbering procedures. At the same time it is important to stress that renumbering a subscriber involves more than just host renumbering. Connecting a subscriber to the Internet by some form of mediating gateway (e.g., application layer gateways, Network Address Translators (NATs), significantly reduces the complexity of renumbering. When such a subscriber changes its providers, the impact of renumbering is usually confined to the mediating gateways used by the subscriber. Although one could view connectivity by mediating gateways as less preferable than pure IP layer connectivity, it is important to stress that either alternative has its own trade-offs. The choice between the alternatives should be based on careful rational evaluation of the trade-offs versus connectivity requirements, and is likely to vary from one enterprise to another. Previous Table of Contents Next 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. 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