IPv6: BGP Classless Interdomain Routing

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Classless Interdomain Routing

The invention of autonomous systems and exterior routing protocols solved the early scalability problems on the Internet in the 1980s. H by the early 1990s the Internet was beginning to present a different set of scalability problems, including the following:

  • Explosion of the Internet routing tables. The exponentially growing routing tables were becoming increasingly unmanageable both routers of the time and the people who managed them. The mere size of the tables was burden enough on Internet resources, but day topological changes and instabilities added heavily to the load.
  • Depletion of the Class B address space. In January 1993, 7133 of the 16,382 available Class B addresses had been assigned; at growth rates, the entire Class B address space would be depleted in less than 2 years (as cited in RFC 1519).
  • The eventual exhaustion of the entire 32-bit IP address space.

Classless interdomain routing provides a short-term solution to the first two problems. Another short-term solution is network address tra (NAT), discussed in Chapter 4, "Network Address Translation." These solutions were intended to buy the Internet architects enough time a new version of IP with enough address space for the foreseeable future. That initiative, known as IP Next Generation (IPng), resulted in creation of IPv6, with a 128-bit address format. IPv6, discussed in Chapter 8, "IP Version 6," is the long-term solution to the third problem

Interestingly, CIDR and NAT have been so successful that few people place as much urgency on the migration to IPv6 as they once did. CIDR is merely a politically sanctioned address summarization scheme that takes advantage of the hierarchical structure of the Internet. discussing CIDR further, a review of summarization and classless routing, and a look at the modern Internet, are in order.

A Summarization Summary

Summarization or route aggregation (discussed extensively in Routing TCP/IP, Volume I) is the practice of advertising a contiguous set o addresses with a single, less-specific address. Basically, summarization/route aggregation is accomplished by reducing the length of the mask until it masks only the bits common to all the addresses being summarized. In Figure 2-1, for example, the four subnets (172.16.10,, and are summarized with the single aggregate address

Figure 2-1. Route Aggregation

Many networkers who view summarization as a difficult topic are surprised to learn that they use summarization daily. What is a subnet a after all, other than a summarization of a contiguous group of host addresses? For example, the subnet address is theaggregate of host addresses through (The "host address" is, of course, the addre data link itself.) The key characteristic of a summary address is that its mask is shorter than the masks of the addresses it is summarizing ultimate summary address is the default address,, commonly written as just 0/0. As the /0 indicates, the mask has shrunk until n network bits remain​ the address is the aggregate of all IP addresses. Summarization can also cross class boundaries. For example, the four Class C networks (,,, and can all be summarized with the aggregate address Notice that the aggregate, with its 22-bit mask, is no lon legal Class C address. Therefore, to support the aggregation of major class network addresses, the routing environment must be classle

Classless Routing

Classless routing features two aspects:

  • Classlessness can be a characteristic of a routing protocol.
  • Classlessness can be a characteristic of a router.

Classless routing protocols carry, as part of the routing information, a description of the network portion of each advertised address. The portion of a network address is commonly referred to as the address prefix. An address prefix can be described by including an address length field that indicates how many bits of the address are prefix bits, or by including only the prefix bits in the update (see Figure 2-2). T classless IP routing protocols are RIP-2, EIGRP, OSPF, Integrated IS-IS, and BGP-4. Figure 2-2. Advertising an Address Prefix with a Classless Routing Protocol A classful router records destination addresses in its routing table as major class networks and subnets of those networks. When it perfo route lookup, it first looks up the major class network address and then tries to find a match in its list of subnets under that major address classless router ignores address classes and merely attempts a "longest match." That is, for any given destination address, it chooses th that matches the most bits of the address. Take the routing table of Example 2-1, for instance, which shows several variably subnetted IP networks. If the router is classless, it attempts to find the longest match for each destination address.

Example 2-1 A Routing Table Containing Several Variably Subnetted IP Networks

Cleveland#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
Gateway of last resort is to network E2 [110/20] via,  00:11:19, Ethernet0
O [110/74] via, 00:11:19, Serial0
O E2 [110/40] via, 00:11:19, Ethernet1 is variably subnetted, 3 subnets, 3 masks
O E1
[110/139] via, 00:11:20, Serial1
O E1
[110/139] via, 00:00:34, Serial1
O E2
[110/20] via, 00:11:20, Ethernet0 is variably subnetted, 4 subnets, 2 masks
C is directly connected, Ethernet0
C is directly connected, Ethernet1
C is directly connected, Serial0
C is directly connected, Serial1
O E2 [110/20] via, 00:11:20, Ethernet0
O E2 [110/20] via, 00:11:21, Ethernet0
O E2 [110/20] via, 00:11:21, Ethernet0
O E2 [110/20] via, 00:11:21, Ethernet0
O E2 [110/20] via, 00:11:21, Ethernet0
O E2 [110/20] via, 00:11:21, Ethernet0
O E2 [110/20] via, 00:11:21, Ethernet0
O*E2 [110/1] via, 00:11:21, Serial0
O E2 [110/40] via, 00:11:22, Ethernet1

If the router receives a packet with a destination address of, several entries in the routing table match the address: 192.168,, and The entry is chosen (see Example 2-2) because it matches 26 bi destination address​ the longest match.

Example 2-2 A Packet with a Destination Address of Is Forwarded Out Interface S1

Cleveland#show ip route
Routing entry for
Known via "ospf 1", distance 110, metric 139, type extern 1Redistributing via ospf 1
Last update from on Serial1, 06:46:52 ago
Routing Descriptor Blocks:
*, from, 06:46:52 ago, via Serial1
Route metric is 139, traffic share count is 1

A packet with a destination address of will not match, nor will it match The longest match address is, as demonstrated in Example 2-3.

Example 2-3 The Router Cannot Match to a More-Specific Subnet, So It Matches the Network Add

Cleveland#show ip route
Routing entry for
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 10
Redistributing via ospf 1
Last update from on Ethernet0, 06:48:18 ago
Routing Descriptor Blocks:
*, from, 06:48:18 ago, via Ethernet0
Route metric is 20, traffic share count is 1

The longest match that can be made for destination address is the aggregate address, as demonstrated in 4.

Example 2-4 Packets Destined for Do Not Match a More-Specific Subnet or Network, and Therefore the Supernet

Cleveland#show ip route
Routing entry for, supernet
Known via "ospf 1", distance 110, metric 139, type extern 1
Redistributing via ospf 1
Last update from on Ethernet1, 06:49:26 ago
Routing Descriptor Blocks:
*, from, 06:49:26 ago, via Ethernet1
Route metric is 139, traffic share count is 1

Finally, a destination address of will not match any of the network entries in the routing table, as demonstrated in Example 2

However, packets with this destination address are not dropped, because the routing table of Example 2-1 contains a default route. TheExample 2-5 No Match Is Found in the Routing Table for; Packets Destined for This Address Are Fo to the Default Address, Out Interface S0

Cleveland#show ip route
% Network not in table

Beginning with IOS 11.3, Cisco routers are classless by default. Prior to this release, the IOS defaults were classful. You can change the with the ip classless command.

The routing table in Example 2-1 and the associated examples demonstrates another characteristic of longest-match routing. Namely, a an aggregate address does not necessarily point to every member of the aggregate. Figure 2-3 shows the vectors of the routes in Examp through 2-5.

Figure 2-3. The Vectors of Routes in the Routing Table of Example 2-1

You can consider network an aggregate of all its subnets; Figure 2-3 shows that the route to this network address directs out interface E0. Yet routes to two of its subnets, and, point out a different interface, S1.

NOTE In fact, is itself a member of The fact that there are distinct routes for these two addresses, both pointin hints that they are advertised by separate routers somewhere upstream. Likewise, is a member of the aggregate, but the route to that less-specific address is out E1. The least-sp route,, which is an aggregate of all other addresses, is out S0. Because of longest-match routing, packets to subnets 192.168.1 and are forwarded out S1, whereas packets to other subnets of network are forwarded out E0. Packets w destination addresses beginning with 192.168, other than 192.168.1, are forwarded out E1, and packets whose destination addresses do begin with 192.168 are forwarded out S0. Summarization: The Good, the Bad, and the Asymmetric Summarization is a great tool for conserving network resources, from the amount of memory required to store the routing table to the am network bandwidth and router horsepower necessary to transmit and process routing information. Summarization also conserves networ resources by "hiding" network instabilities. For example, the network in Figure 2-4 has a flapping route​ a route that, due to a bad physical connection or router interface, keeps trans down and up and down again.

Figure 2-4. A Flapping Route Can Destabilize the Entire NetworkWithout summarization, every time subnet goes up or down, the information must be conveyed to every router in the c internetwork. Each of those routers, in turn, must process the information and adjust its routing table accordingly. If router Nashville adve the upstream routes with the aggregate address, however, changes to any of the more-specific subnets are not advert that router. Nashville is the aggregation point; the aggregate continues to be stable even if some of its members are not. The price to be paid for summarization is a reduction in routing precision. In Example 2-6, interface S1 of the router in Figure 2-3 has fail causing the routes learned from the neighbor on that interface to become invalid. Instead of dropping packets that would normally be forw out S1, however, such as a packet with a destination address of, the packet now matches the next-best route, forwarded out interface E0. (Compare this to Example 2-2.)

Example 2-6 A Failed Route Can Lead to Inaccurate Packet Forwarding

%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down
%LINK-3-UPDOWN: Interface Serial1, changed state to down
Cleveland#show ip route
Routing entry for
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 10
Redistributing via ospf 1
Last update from on Ethernet0, 00:00:20 ago
Routing Descriptor Blocks:
*, from, 00:00:20 ago, via Ethernet0
Route metric is 20, traffic share count is 1

This imprecision may or may not be a problem, depending on what the rest of the internetwork looks like. Continuing with the example, sthe next-hop router still has a route entry to via the router Cleveland, either because the internetwork has n converged or because the route was statically entered. In this case, a routing loop occurs. On the other hand, some router reachable via

Cleveland's E0 interface may have a "back door" route to subnet that should be used only if the primary route, via Cleve becomes invalid. In this second case, the route to has been designed as a backup route, and the behavior shown in Exam is intentional.

Figure 2-5 shows an internetwork in which a loss of routing precision can cause a different sort of problem. Here, routing domain 1 is con routing domain 2 by routers in San Francisco and Atlanta. What defines these domains is unimportant for the example. What is importan the networks in domain 1 can be summarized with the address, and all the networks in domain 2 can be summarized w address

Figure 2-5. When Multiple Routers Are Advertising the Same Aggregate Addresses, Loss of Routing Precisio Become a Problem

Rather than advertise individual subnets, Atlanta and San Francisco advertise the summary addresses into the two domains. If a host on subnet sends a packet to a host on Seattle's subnet, the packet most likely is routed to Atlanta, bec is the closest router advertising domain 2's summary route. Atlanta forwards the packet into domain 2, and it arrives at Seattle. When the subnet sends a reply, Seattle forwards that packet to San Francisco​ the closest router advertising the summary route The problem here is that the traffic between the two subnets has become asymmetric: Packets from to one path, whereas packets from to take a different path. Asymmetry occurs because the Dallas an routers do not have complete routes to each other's subnets. They have only routes to the routers advertising the summaries and must fo packets based on those routes. In other words, the summarization at San Francisco and Atlanta has hidden the details of the internetwor those routers.

Asymmetric traffic can be undesirable for several reasons. First, internetwork traffic patterns become unpredictable, making baselining, c planning, and troubleshooting more problematic. Second, link usage can become unbalanced. The bandwidth of some links can become saturated, while other links are underutilized. Third, a distinct variation can occur in the delay times of outgoing traffic and incoming traffic delay variation can be detrimental to some delay-sensitive applications such as voice and live video. The Internet: Still Hierarchical After All These Years Although the Internet has grown away from the single-backbone architecture of the ARPANET described in Chapter 1, it retains a certain hierarchical structure. At the lowest level, Internet subscribers connect to an Internet service provider (ISP). In many cases, that ISP is on many small providers in the local geographic area (called local ISPs). For example, there are presently almost 200 ISPs in Colorado's 30 code. These local ISPs in turn are the customers of larger ISPs that cover an entire geographic region such as a state or a group of adja states. These larger ISPs are called regional service providers. Examples in Colorado are CSD Internet and Colorado Supernet. The reg service providers, in turn, connect to large ISPs with high-speed (DS-3 or OS-3 or better) backbones spanning a national or global area. largest providers are the network service providers and include companies such as MCI/WorldCom (UUNET), SprintNet, Cable & Wirele Concentric Network, and PSINet. More commonly, these various providers are referred to as Tier III, Tier II, and Tier I providers, respecti Figure 2-6 shows how these different types of ISPs are interrelated. In each case, a subscriber​ whether an end user or a lower-level servprovider​ connects to a higher-level service provider at that ISP's Point of Presence (POP). A POP is just a nearby router to which the sub can connect via dialup or a dedicated local loop. At the highest level, the network service providers interconnect via network access poin A NAP is a LAN or switch​ typically Ethernet, FDDI, or ATM​ across which different providers can exchange routes and data traffic. Figure 2-6. ISP/NAP Hierarchy As Table 2-1 shows, some NAPs are known by names such as Commercial Internet Exchange (CIX), Federal Internet Exchange (FIX), a Metropolitan Area Exchange (MAE​ originally called Metropolitan Area Ethernets, a creation of Metropolitan Fiber Systems, Inc.). CIX, FIX MAE-East were early experiments to connect backbones; based on the experience gained from these connection points, the National Sc Foundation implemented the first four NAPs in 1994 as part of the decommissioning of the NSFnet.

New York NAP [*]
Chicago NAP [*]
San Francisco NAP [*]
Big East NAP
MAE-East [*]
MAE-New York
Digital PAIX

Table 2-1. Well-Known Network Access Points in the United States

Maintained By
Pennsauken, New Jersey
Chicago, Illinois
San Francisco, California Sprint
Ameritech and Bellcore
Pacific Bell
Bohemia, New York
San Jose, California
Washington, DC ICS Network Systems
Los Angeles, California
Houston, Texas
Dallas, Texas
New York City, New York
Chicago, Illinois
College Park, Maryland
Moffett Field, California
Santa Clara, California
Palo Alto, California MCI/WorldCom
University of Maryland
NASA Ames Research Center
Digital Equipment Corporation

One of the original four NSF NAPs In addition to the major NAPs shown in Table 2-1, where the NSPs come together, there are many smaller NAPs. These usually intercon smaller regional providers. Examples of regional NAPs are Seattle Internet eXchange (SIX) and the New Mexico network access point. In conjunction with the formation of the NAPs, the NSF funded the Routing Arbiter (RA) project. One of the duties of the RA is to promote stability and manageability. To this end, the RA proposed a database (the RADB, or Routing Arbiter Database) of routes (topology) and p (preferred paths) from the service providers. The database is maintained at NAPs on a route server, a UNIX workstation or server runnin Rather than peering with every other router at the NAP, each provider's router peers with only the route server. Routes and policies arecommunicated to the server, which uses a sophisticated database language called RIPE-181 to process and maintain the information. Th appropriate routes are then passed to the other routers. Although the route server speaks BGP and processes routes, it does not perform packet forwarding. Instead, its updates inform routers o next-hop router that is directly reachable across the NAP. You are already familiar with this concept from the discussion in Chapter 1 of E party neighbors. By making one-to-many peering feasible rather than many-to-many peering, route servers increase the stability, manage and throughput of traffic through the NAPs. The NAPs and the RA project proved that the competing network service providers could cooperate to provide manageable connectivity stability to the Internet. As a result, the NSF ceased funding of the route servers and NAPs on January 1, 1997, and turned the operation the commercial interests. Although publicly funded Internet research continues with such projects as Internet2, GigaPOPs, and the very speed Backbone Network Service (vBNS), the present Internet can be considered a commercial operation. A result of the transition to commercial control of the Internet is that the topology of the modern Internet is far from the tidy picture drawn preceding paragraphs. The largest service providers, driven by financial, competitive, and policy interests, generally choose to peer direc than peer through route servers. The peering also takes place at many levels, rather than just at the top level shown in Figure 2-6. When two or more service providers agree to share routes across a NAP, either directly or through a route server, they enter into a peerin agreement. A peering agreement may be established directly between two providers (a bilateral peering agreement) or between a group sized providers (a multilateral peering agreement, or MLPA). Traffic patterns play a major role in determining the financial nature of the a If the traffic between the peering partners is reasonably balanced in both directions, money usually does not exchange hands. The peerin equitable for the two partners. However, if the traffic is heavier in one direction than in the other across the peering point, as is the case w small provider peers with a larger provider, the small provider usually must pay for the peering privilege. The rationale here is that the sm provider benefits more from the peering than the larger provider. Another factor muddling the Internet picture is the location of peering points. NAPs in which many providers come together, such as the o in Table 2-1, are public peering sites. In addition to these public sites, service providers have created hundreds of smaller NAPs at sites w they find themselves co-located with other service providers. The peering agreements at such sites are usually private agreements betw a few providers. Private peering is encouraged because it helps relieve congestion at the national NAPs, adds to route diversity, and can delay for some traffic.

Another fact hinting that real life is not as tidy as Figure 2-6 suggests is that many national and regional service providers also sell local I access, in direct competition with the local ISPs. The "starting point" of the route traces in Example 2-7, for example, is a dial-in POP bel Concentric Network​ a backbone provider. Regional service providers also frequently have a presence at the backbone NAPs. They might to one or more network service providers across the NAP, or they might connect to other regional service providers across the NAP, bypa network service provider. The route traces in Example 2-7 show a little of the Internet backbone structure. Both traces originated from a Concentric Network POP i In the first trace, the packets traverse Concentric Network's backbone to MAE-East, where they connect to the BBN Planet backbone (lin 4). The packets traverse BBN Planet's backbone to a Tier II NAP shared by BBN and US West in Minneapolis (lines 10 and 11) and then passed to the US West destination.

Example 2-7 Route Traces from a Concentric Network POP in Denver

--- traceroute to www.uswest.com (,
30 hops max, 18 byte packets
2 (
174 ms
162 ms
3 ( us-dc-wash-core1-a1-0d12.rtr.concentric.net
4 ( maeeast2.bbnplanet.net
5 ( 6 ( p3-1.nyc4-nbr2.bbnplanet.net
222 ms
7 ( p1-0.nyc4-nbr3.bbnplanet.net
223 ms
225 ms
232 ms
385 ms8 (
9 ( p10-0-0.chicago1-br1.bbnplanet.net
10 ( h1-0.minneapol1-cr1.bbnplanet.net
11 ( 12 (
13 ( www.uswc.uswest.net
235 ms
239 ms
258 ms
260 ms
249 ms
258 ms
--- traceroute to www.rmi.net (,
30 hops max, 18 byte packets
1 ( ts003e01.den-co.concentric.net
152 ms
2 ( rt001e0102.den-co.concentric.net.168.155.207.IN-ADDR.ARPA
161 ms
3 (
190 ms
4 ( us-ca-scl-core1-f9-0.rtr.concentric.net
5 ( 6 ( sl-gw18-chi-5-1-0-T3.sprintlink.net
7 ( sl-bb11-chi-3-3.sprintlink.net 8 ( sl-bb5-chi-4-0-0.sprintlink.net 211  ms
9  ( sl-bb7-pen-5-1-0.sprintlink.net 225 ms
10 ( sl-bb10-pen-1-3.sprintlink.net 11 ( sl-nap1-pen-4-0-0.sprintlink.net 12 ( 13 ( f1-1.t60-6.Reston.t3.ans.net 14 ( h12-1.t64-0.Houston.t3.ans.net 15 ( h13-1.t80-1.St-Louis.t3.ans.net 16 ( h14-1.t24-0.Chicago.t3.ans.net 17 ( h14-1.t96-0.Denver.t3.ans.net 309 ms
18 ( f1-0.c96-10.Denver.t3.ans.net 313 ms
19 ( h1-0.enss3191.t3.ans.net 20 ( 21 (
206 ms
189 ms
210 ms
216 ms
236 ms
228 ms
263 ms
264 ms
286 ms
283 ms
292 ms
306 ms
305 ms
285 ms

The packets in the second trace take a pretty thorough tour of the United States before arriving at their destination, a few miles from theiorigination. First, they follow Concentric's backbone through a router in California (line 4) and then to the Chicago NAP, where they conne Sprint backbone (line 6). The packets are routed to the New York NAP in Pennsauken, New Jersey, where they are passed to the ANS b (lines 11 and 12). They then visit routers in Reston, Houston, St. Louis, and Chicago (again), and finally arrive back in Denver. Like the packets in the last trace, we have taken a rather lengthy and circuitous route to get back to the topic at hand, CIDR. CIDR: Reducing Routing Table Explosion Given the somewhat hierarchical structure of the Internet, you can see how the structure lends itself to an address summarization schem top layers, large blocks of contiguous Class C addresses are assigned by the Internet Assigned Numbers Authority (IANA) to the various addressing authorities around the globe, known as the regional IP registries. Currently, there are three regional registries. The regional re North and South America, the Caribbean, and sub-Saharan Africa is the American Registry for Internet Numbers (ARIN). ARIN also is re for assigning addresses to the global network service providers. The regional registry for Europe, the Middle East, northern Africa, and p Asia (the area of the former Soviet Union) is the Resèaux IP Europèens (RIPE). The regional registry for the rest of Asia and the Pacific the Asia Pacific Network Information Center (APNIC).

NOTE ARIN was spun off of the InterNIC (run by Network Solutions, Inc.) in 1997 to separate the management of IP addresses and domain nam

Table 2-2 shows the original scheme for assigning Class C addresses to the regions these registries serve, although some of the allocati now outdated. As Example 2-8 demonstrates, the blocks labeled "Others" are now being assigned. The regional registries, in turn, assign of these blocks to the large service providers or to local IP registries. Generally, the blocks assigned at this level are no smaller than 32 c Class C addresses (and are usually larger). Concentric Network has been assigned the block, for example, which inclu equivalent of 128 contiguous Class C addresses (see Example 2-8).

Table 2-2. CIDR Address Allocation by Geographic Region

Address Range
North America
Central/South America
Pacific Rim

Example 2-8 When a WHOIS Is Performed on the Address from Example 2-7, the Address Is Show Part of a /17 CIDR Block Assigned to Concentric Network

--- looking up
--- performing WHOIS on "", please wait...
--- contacting host whois.arin.net
--- smart query on "207.155.128"
Concentric Research Corp. (NETBLK-CONCENTRIC-CIDR)
10590 N. Tantau Ave.Cupertino, CA
Netblock: -
Maintainer: CRC
(408) 342-2800
Fax- (408) 342-2810
Domain System inverse mapping provided by:
Record last updated on 13-Feb-97.
Database last updated on 29-Jan-99 16:12:40 EDT.

The service providers receiving these blocks assign them in smaller blocks to their subscribers. If those subscribers are themselves ISPs again break their blocks into smaller blocks. The obvious advantage of assigning these blocks of Class C addresses, called CIDR blocks when the blocks are summarized back up the hierarchy. For more information on how addresses are assigned throughout the Internet, s 2050 (www.isi.edu/in-notes/rfc2050.txt). To illustrate, suppose Concentric Network assigns to one of its subscribers a portion of its block, consisting of 207.155 If that subscriber is an ISP, it may assign a portion of that block, say, to one of its own subscribers. That subscriber adv /22 (read "slash twenty-two") block back to its ISP. That ISP in turn summarizes all of its subscribers to Concentric Network with the singl aggregate, and Concentric Network summarizes its subscribers into the NAPs to which it is attached with the single ag The advertisement of a single aggregate to the higher-level domain is obviously preferable to advertising possibly hundreds of individual addresses. But an equally important benefit is the stability such a scheme adds to the Internet. If the state of a network in a low-level dom changes, that change is felt only up to the first aggregation point and no further. Table 2-3 shows the different sizes of CIDR blocks, their equivalent size in Class C networks, and the number of hosts each block can re Table 2-3. CIDR Block Sizes Number of Equivalent Class C Addresses CIDR Block Prefix Size /24 /23 /22 /21 /20 1 2 4 8 16 Number of Possible Host Addre 254 510 1022 2046 4094/19 /18 /17 /16 /15 /14 /13 32 64 128 256 512 1024 2048 8190 16,382 32,766 65,534 131,070 262,142 524,286 CIDR: Reducing Class B Address Space Depletion The depletion of Class B addresses was due to an inherent flaw in the design of the IP address classes. A Class C address provides 254 addresses, whereas a Class B address provides 65,534 host addresses. That's a wide gap. Before CIDR, if your company needed 500 h addresses, a Class C address would not have served your needs. You probably would have requested a Class B address, even though y be wasting 65,000 host addresses. With CIDR, your needs can be met with a /23 block. The host addresses that would have otherwise b wasted have been conserved. Difficulties with CIDR Although CIDR has proven successful in slowing both the growth of Internet routing tables and the depletion of Class B addresses, it also presented some problems for the users of CIDR blocks. The first problem is one of portability. If you have been given a CIDR block, the addresses are most likely part of a larger block assigned ISP. Suppose, however, that your ISP is not living up to your expectations or contractual agreements, or you have just gotten a more attr offer from another ISP. A change of ISPs most likely means you must re-address. It's unlikely that an ISP will allow a subscriber to keep i assigned block when the subscriber moves to a new provider. Aside from an ISP's being unwilling to give away a portion of its own addre regional registries strongly encourage the return of address space when a subscriber changes ISPs. For an end user, re-addressing carries varying degrees of difficulty. The process is probably the easiest for those who use private addres within their routing domain and network address translation (see Chapter 4) at the edges of the domain. In this case, only the "public-faci addresses have to be changed, with minimal impact on the internal users. At the other extreme are end users who have statically assign addresses to all their internal network devices. These users have no choice but to visit every device in the network to re-address. Even if the end user is using the CIDR block throughout the domain, the pain of re-addressing can be somewhat reduced by the use of D BOOTP). In this case, the DHCP scopes must be changed and users must reboot, but only some statically addressed network devices, s servers and routers, must be individually re-addressed. The problem is much amplified if you are an ISP rather than an end user and you want to change your upstream service provider. Not on your own internetwork be renumbered, but so must any of your subscribers to whom you have assigned a portion of your CIDR block. CIDR also presents a problem to anyone who wants to connect to multiple service providers. Multihoming (discussed in more depth later chapter) is used for redundancy so that an end user or ISP is not vulnerable to the failure of a single upstream service provider. The trou if your addresses are taken from one ISP's block, you must advertise those addresses to the second provider. Figure 2-7 shows what can happen. Here, the subscriber has a /23 CIDR block that is part of ISP1's larger /20 block. When the subscribe attaches to ISP2, he wants to ensure that traffic from the Internet can reach him through either ISP1 or ISP2. To make this happen, he m advertise his /23 block through ISP2. The trouble arises when ISP2 advertises the /23 block to the rest of the world. Now all the routers " have a route to advertised by ISP1 and a route to advertised by ISP2. Any packets destined for the sub are forwarded on the more-specific route, and as a result, almost all traffic from the Internet to the subscriber is routed through ISP2​ inclu from sources that are geographically much closer to the subscriber through ISP1. Figure 2-7. Incoming Internet Traffic Matches the Most-Specific RouteIn Figure 2-7, it is even possible for the route to be advertised into ISP1 from the Internet. This shouldn't happen, becau ISPs set route filters to prevent their own routes from reentering their domain. However, there are no guarantees that ISP1 is filtering pro the more-specific route should leak in from the Internet, traffic from ISP1's other subscribers could traverse the Internet and ISP2 to rather than take the more-direct path. For the subscriber to be multihomed, ISP1 must advertise the more-specific route in addition to its own CIDR block (see Figure 2-8). Mos providers will not agree to this arrangement, because it means "punching a hole" in their own CIDR block (sometimes called address lea addition to reducing the overall effectiveness of CIDR, advertising a more-specific route of its own CIDR block carries an administrative b the ISP. Figure 2-8. ISP1 "Punches a Hole" in Its CIDR Block Although Figures 2-7 and 2-8 show ISP1 as having only a single connection to the Internet, in most cases an ISP has many connectionslevel providers and at NAPs. At each of these connections, the provider must reconfigure its router to advertise the more-specific route in to the CIDR block, and possibly must modify all its incoming route filters. Administration is also complicated by the fact that ISP1 and ISP closely coordinate their efforts to ensure that the subscriber's /23 block is advertised correctly. Because ISP1 and ISP2 are competitors, both might be resistant to working so closely together. Even if the subscriber in Figure 2-8 can get ISP1 and ISP2 to agree to advertise its own /23 block, there is another obstacle. Some Tier I accept only prefixes of /19 or smaller, to control the backbone-level routing tables. If ISP1 or ISP2 or both get their Internet connectivity fr these network service providers, they cannot advertise the subscriber's /23. The practice of filtering any CIDR addresses with a prefix lar /19 has become so well-known that a /19 prefix is commonly referred to as a globally routable address. The implication here is that if you a longer CIDR prefix, say a /21 or /22, your prefix might not be advertised to all parts of the Internet. Remember that any parts of the Inte do not know how to reach you are essentially unreachable by you. NOTE Many Tier I providers have relaxed their /19 rules recently in response to increased subscriber complaints. A possible solution for the multihomed subscriber in Figure 2-8 is to obtain a provider-independent address space (also known as a porta address space). That is, the subscriber can apply for a block that is not a part of either ISP1's or ISP2's CIDR block; both ISPs can adver subscriber's block without interference with their own address space. Since the formation of ARIN, obtaining a provider-independent bloc somewhat easier than it was under the InterNIC. Although ARIN strongly encourages you to seek an address space first from your provid second from your provider's provider, obtaining a provider-independent address space from ARIN is a last resort. However, you still face difficulties. First, if you want to multihome, it is likely that your present address space was obtained from your original ISP. Changing to a provider-in address space means renumbering, with all the difficulties already discussed. (Of course, if you obtained your IP address space in the pr days, you are already provider-independent, making the question moot.) Second, the registries assign address space based on justified need, not on long-term predicted need. This policy means that you proba allocated "just enough" space to fit your present needs and a three-month predicted need. From there, you have to justify a further alloca proving that you are efficiently using the original space. For example, ARIN requires proof of address utilization by one of two means: the the Shared WHOIS Project (SWIP) or the use of a Referral WHOIS Server (RWHOIS). SWIP, most commonly used, is the practice of ad WHOIS information to a SWIP template and e-mailing it to ARIN. To use RWHOIS, you establish an RWHOIS server on your premises th can access for WHOIS information. In both cases, the WHOIS information establishes proof that you have efficiently used, and are appro exhaustion of, your present address space. Of course, you still have a problem if you cannot justify obtaining a globally routable (/19) address space. The bottom line is that CIDR al rules make multihoming a difficult problem for small subscribers and ISPs. The following section discusses multihoming in more detail, a some alternative topologies.

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