Variabilidad en la prescripción y consumo:

■ _{The Loopback Address}

■ _{The Unspecified Address}

■ Interface Identifiers

■ _{Local-use Unicast Addresses}

■ _{NSAP Addresses}

■ _{IPX Addresses}

Aggregatable Global Unicast Address

The Aggregatable Global Unicast address format is used to define a tiered structure for the assignment and allocation of the IPv6 address space.The new structure calls for the address space to be broken into six separate components.These com- ponents are the Format Prefix (FP), the Top-Level Aggregation Identifier (TLA ID), Reserved (RES), Next-Level Aggregation Identifier (NLA ID), Site-Level Aggregation Identifier (SLA ID), and the Interface Identifier (Interface ID). Figure 4.2 depicts the global unicast address format.

128 Chapter 4 • Explaining IPv6 Addressing

The public routing topology prefix consists of the first 48 bits of unicast address space.This includes the FP bits plus the TLA, Reserved, and NLA bits. The next 16 bits define the site topology and the final 64 bits are the interface identifier.The public topology is address space assigned to exchanges and ISPs that provide transit Internet services.The site topology can be defined as the cus- tomers of the providers and exchanges and the interface identifier is an end host or node interface identifier.

FP The Format Prefix for Globally Routable Unicast prefixes will always

have the same three bits (in the initial deployment of IPv6).These first three bits will always be set to 001 and are there to designate (to any routing entity on the Internet) that this address is a Globally Routable Unicast address. For each type of IPv6 address that we discuss, the FP will be unique to that type of address, thus making it easier for routing entities to discern packet types and to process them according to the rules that apply to the respective packet type. For example, multicast packets and unicast packets are routed in very different ways. Unicast packet routing is 1-to-1 (a packet with an IPv6 Globally Routable Unicast destination originates from one host and is delivered to one host), and multicast packets are 1-to-N (one multicast packet may be delivered to N interested destination hosts), or N-to-N (N sources deliv- ering packets to N destinations), so these packets are handled in vastly different ways on an Internet backbone.The FP serves as a delimiter, so a routing device can make a quick decision as to how to handle the incoming packet and ensure that it is handled correctly. Note that using the first few bits of an address to designate type of address is more effi- cient than putting it into the packet, because now we can utilize more of the packet for other valuable features, discussed earlier.

TLA ID The TLA ID utilizes 13 bits that provide for 8,192 TLAs.This means there can be 8192 providers or exchanges at this level.This is analogous to today’s Tier-1 providers.The TLAs reside at the highest

Figure 4.2 Global Unicast Address Format

001 TLA ID RES NLA ID SLA ID Interface ID 13 bits 8 bits 24 bits 16 bits 64 bits 3 bits

Explaining IPv6 Addressing • Chapter 4 129 point of the routing hierarchy.TLAs will be assigned one of the 8,192

TLA IDs and will own responsibility for allocating addresses to down- stream customers.

N

The Internet community discusses intently what defines a Tier-1 provider. Possibly the simplest definition is that these are providers that do not pay peering costs to exchange routing information with each other. However, no standard definition exists, nor are there any rules governing who can peer with whom. This lack of standards and rules leads to the question of who will be TLA providers and who will make this decision. This is important in terms of which providers will be classified as TLAs and own one of the 8,192 address blocks at the TLA level and how they plan their address allocations.

RES These bits are reserved for now. It has not been determined by the IETF what course of action should be used for these bits. At this stage, it is appropriate for TLAs to subnet their assignment using these 8 bits to increase the amount of Globally Routable Unicast address space that a TLA can use to delegate to their customers or use on their Backbone.

NLA ID These 24 bits depict the Next-Level Aggregator Identifier. A Next-Level Aggregator can be thought of today as a Tier-2 network ser- vice provider or ISP. An NLA can range from a small organization with one TLA connection, to a large, regional provider with many upstream TLA connections and complex backbones. An NLA will receive an NLA ID from its upstream TLA, and, in turn, will break its NLA ID into chunks, which will be delegated to its customers.

SLA ID A Site-Level Aggregator Identifier describes an entity that has no downstream customers who are network service providers. An SLA could be a small to large business, or a small service provider who does not delegate address space to its providers (for instance, today’s cable- modem providers could fit into an SLA arrangement).

Interface ID The final 64 bits of the Globally Routable IPv6 Unicast address is reserved for the Interface Identifier. In IPv4 terms, this is known as the host ID.These 64 bits will be designated to distinguish one host

130 Chapter 4 • Explaining IPv6 Addressing

from another on a given network segment. Each Interface ID on a given network segment must be unique.We will see that IPv6 builds in a clever way to ensure this is so.

To effectively understand IPv6 addressing, it is necessary to understand how the address space is allocated.The policies governing the assignment and alloca- tion of the IPv6 address space have been set forth in a published standard entitled ripe-196.This abstract defines the Internet registry system for the distribution of IPv6 unicast addresses.The goal is to ensure that the IPv6 address space is man- aged properly.This management is necessary to eliminate the inconsistencies and unfairness of address allocation seen in the IPv4 address space.This minimizes waste of address space and maximizes aggregation.The authority for all IPv6 address space is the Internet Assigned Numbers Authority (IANA). IANA allo- cates IPv6 address space to regional Internet Registries (IRs).There are three IRs: ARIN, RIPE Network Cordination Centre (NCC), and APNIC. ARIN serves North and South America, the Caribbean, and sub-Saharan Africa; RIPE NCC handles Europe, the Middle East, and some parts of Africa; APNIC handles areas in the Asia Pacific region.The IRs can handle areas outside their adminis- trative control if necessary. Additional IRs can be created as the deployed base of IPv6 addresses grows.The policies governing the assignment of IPv6 unicast address space must be done in a manner that ensures that each unicast address is efficiently allocated, globally routable, unique, and supports aggregation. RFC 2374 (the update to RFC 2373) organizes the IPv6 address space into a topolog- ical hierarchy.The topologies are public topology, site topology, and interface. Figure 4.3 depicts the mapping of these topologies to the IPv6 address space.

An IPv6 address can be expressed in the same manner as IPv4 addresses. An IPv4 address contains network and host bits.The network and host portions of an IPv6 address are shown in Figure 4.4.

The network portion of an IPv6 address is 64 bits and the host portion is 64 bits.The expression of IPv6 addresses uses CIDR notation to divide the address

Figure 4.3 Unicast Address Hierarchical Topology

001 TLA ID RES NLA ID SLA ID Interface ID

Public Topology 48 bits Site Topology 16 bits Interface Topology 64 bits 13 bits 8 bits 24 bits 16 bits 64 bits 3 bits

Explaining IPv6 Addressing • Chapter 4 131 into network and host portions. An address with a /48 mask represents an aggre-

gatable network prefix assigned out of the public topology allocation. Subnetting and address assignment are discussed in detail later in this chapter.

The regional IRs use a modified version of the IPv6 address TLA field for allocation of the initial address space.This requires dividing the TLA space into a sub-TLA, which allows for allocating less of the address space to the original TLAs as previously planned.The IR reserves the additional 6 bits for the TLA so that if the need arises, it can allocate the address space to the TLA.The reserva- tion preserves the aggregation policies but allows for efficient allocation on an as- needed basis.This policy creates a modified format to the IPv6 address space.The modified version is shown in Figure 4.5.

The address allocation policy for IPv6 is as follows:

■ _{Regional IRs assign addresses to qualified sub-TLAs (TLA ISPs).}

■ TLA ISPs assign NLA addresses to NLA ISPs (TLA customers).

■ _{NLA ISPs assign SLA addresses to their customers.}

TLA providers have to be able to allocate SLA addresses to customers.TLA ISPs provide direct Internet connectivity to end users as well as NLAs.

Subnetting the address space within the TLA and NLA providers’ networks is left up to the discretion of the individual providers. Regardless of how providers

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Figure 4.4 Unicast Address Network and Host Bits

001 TLA ID RES NLA ID SLA ID Interface ID

Public Topology 48 bits Site Topology 16 bits Interface Topology 64 bits 13 bits 8 bits 24 bits 16 bits 64 bits

Network 64 bits

Host 64 bits 3 bits

Figure 4.5 Modified Sub-TLA Format

001 TLA ID Sub-TLA RES NLA ID SLA ID Interface ID

13 bits 13 bits 6 bits 13 bits 16 bits 64 bits 3 bits

132 Chapter 4 • Explaining IPv6 Addressing

subnet the address space within their networks, aggregation of the address space falls on the prefix boundaries shown in Table 4.3.

Table 4.3 Aggregation Prefix Boundaries

ID Longest Prefix Length in Bits

TLA /16 13

Sub-TLA /29 13

Reserved

N:LA /48 13

SLA /64 16