So far, we have discussed AS numbering, IP addressing, customer and provider relationships, and so on. An important question remains: how does an organization obtain an IP address block? In this section, we answer this question.
Internet Corporation for Assigned Names and Numbers (ICANN) is the organization that handles global coordination of unique identifiers used in the Internet. Through agree- ments, IP address block assignments have been distributed to five different Regional Internet Registries (RIRs). The five RIRs are geographically organized as follows:
• American Registry for Internet Numbers (ARIN) (http://www.arin.net/) to serve the North American region
• RIPE (Réseaux IP Européens) Network Coordination Centre (http://www.ripe.net/) to serve the European and the West Asian region
• Asia Pacific Network Information Centre (APNIC) (http://www.apnic.net/) to serve the South/East Asian and the Pacific region
• Latin American and Caribbean Internet Address Registry (LACNIC) (http://www.lacnic.
net/) to serve the Latin and South American region
• African Network Information Center (AfriNIC) (http://www.afrinic.net/) to serve the African region.
Each registry has its own rules and pricing in regard to IP address block allocation; this allocation depends on allocation size as well, as indicated through netmask boundary such as /19. For example, ARIN’s current policy is that the minimum allocation size is a /20, while for multihomed organizations, the minimum allocation size is a /22. This means that if an organization needs only a /24 allocation, it cannot obtain it directly from ARIN; instead, it
9.4AllocationofIPPrefixesandASNumber
F I G U R E 9.11 AS-based Internet “Skitter” graph generated by Cooperative Association for Internet Data Analysis (CAIDA), 2005. (Copyright c2005. The Regents of the University of California. All Rights Reserved. Used by permission.)
must obtain it from an upstream ISP (provider) who has been already allocated at least a /20 address block by ARIN. Similarly, registries put restrictions on allocation of an AS number.
For example, currently ARIN would allocate an AS number to an organization only if it plans to do multihomed connectivity to two ISPs or can justify that it has a unique routing policy requirement. Note that allocation polices, both for IP prefix and AS number, do change from time to time. For recent polices, you may check the web site of the respective registries.
Suppose that an organization obtains an IP address block along with an AS number from ARIN. It would then need to establish multihomed connectivity to two upstream ISPs who would have their respective AS numbers. Once the physical connectivity is set up, the BGP speaker at the organization establishes a BGP session with the BGP speakers of its upstream ISPs to announce its address blocks. This information is then propagated throughout the Internet so that the rest of the Internet would know how to reach a host in this address block.
Note that the organization may have separate BGP speakers, one each for connecting to its upstream ISPs; in this case, the organization would need to run IBGP between its two BGP speakers in order to establish rules on how to handle routing of outgoing traffic.
Now suppose that an organization obtains an IP prefix from one of the regional Inter- net registries but does not obtain an AS number. In this case, at first it would then need to set up an agreement with an ISP that has an AS number; this ISP would then serve as the
“home” AS for this address block. Once connectivity and agreements are put in place, this ISP would then announce this IP prefix along with other IP prefixes that are in its AS to its upstream provider(s). Once this announcement is propagated throughout the Internet, the newly announced IP prefix becomes known to the rest of the Internet. We discuss below two possibilities of how the connectivity between an organization (customer) and its provider can be set up when a customer does not own a public AS number:
• The ISP may set up private AS numbering to divide its customers into different ASes.
Thus, each customer (organization) has the flexibility to choose a routing protocol of its choice internally and then use a BGP session to talk to the provider’s BGP and announce its IP prefixes.
• If the provider uses OSPF protocol, then it can use Not-so-stubby Areas (NSSA) LSA (refer to Section 6.2.8) to allow external routes from its customer to be advertised into the OSPF autonomous system, while the customer may run its own routing protocol.
Choosing one over another or using any other mechanisms depends on the size of an ISP, as well as its internal network management philosophy and policy, and its business strategy.
Furthermore, note that route redistribution (refer to Section 5.7) is a common mechanism to exchange prefixes among different administrative organizations that fall within an AS num- ber.
It may be noted that a customer who obtains an IP address block from a provider may choose to switch to a different provider after some time. Suppose that a provider has the address block 192.168.40.0/24, and it has allocated 192.168.45.0/24 to a customer. Initially, through route aggregation the provider will announce 192.168.40.0/21 with its AS number.
Now the customer wants to move to a different provider keeping the address block. Thus, the address block, 192.168.45.0/24, would now need to be announced with the AS number of the new provider. This then creates a situation, known as a hole since the more-specific prefix
(192.168.45.0/24) creates a hole in the aggregated prefix (192.168.40.0/21). However, both the aggregated prefix and the more-specific prefix would need to reside in the global routing table at a BGP speaker; this is so that packets can be forwarded properly to the right destination.
This means that the IP address lookup process at a router needs to work very efficiently for handling holes as well. Details on IP address lookup algorithms will be covered later in Chapter 15.