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.MAC addresses, being theoretically unique addresses, can be concatenated with standard IPaddress prefixes to form unique addresses for mobile or transitory users.An example of a link-localaddress with a MAC address is 1111111011:0:mac_address.Site-Local-Use Unicast AddressSite-local addresses are designed for use in a single site.They may be used for sites or organizations thatare not connected to the global Internet.They do not need to request or "steal" an address prefix from theglobal Internet address space.IPv6 site-local addresses can be used instead.When the organizationconnects to the global Internet, it can then form unique global addresses by replacing the site-local prefixwith a subscriber prefix that contains a registry, provider, and subscriber identification.Site-local addresses have the following format:A 10-bit local use flag that is always set to 1111111011A reserved, unnamed field that is n bits in length but defaulted to a value of 0A Subnet ID field that is m bits in lengthAn Interface ID field that is 118 (n+m) bits in lengthAn example of a site-local address is 1111111011:0:subnet:interface.IPv6 Transitional Unicast Address StructuresTwo special IPv6 unicast addresses have been defined as transition mechanisms to allow hosts androuters to dynamically route IPv6 packets over IPv4 network infrastructure and vice versa.IPv4-Compatible IPv6 Unicast AddressThe first unicast address type is called an IPv4-compatible IPv6 address.This transitional unicast addresscan be assigned to IPv6 nodes and can contain an IPv4 address in the last 32 bits.Figure 5-5 illustratesthe format of these addresses.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2905.htm (13 of 15) [02/02/2001 11.37.21] Internet Protocols VersionsFigure 5-5: The structure of an IPv4-compatible IPv6 unicast address.IPv4-Mapped IPv6 Unicast AddressA second, similar type of IPv6 address that also contains an IPv4 address in its last 32 bits is known as anIPv4-mapped IPv6 address.This address is constructed by a dual protocol router and permits IPv4-onlynodes to tunnel through IPv6 network infrastructure.The only difference between IPv4-mapped IPv6addresses and IPv4-compatible IPv6 addresses is that IPv4-mapped addresses are constructs only.Theyare built automatically by dual protocol routers, and cannot be assigned to any nodes.Figure 5-6illustrates the format of these addresses.Figure 5-6: The structure of an IPv4-mapped IPv6 unicast address.Both the IPv4-mapped and the IPv4-compatible unicast addresses are essential to tunneling.Tunnelingenables the transport of packets through an otherwise incompatible network region by wrapping thosepackets in an externally acceptable framework.IPv6 Anycast Address StructuresThe anycast address, introduced in IPv6, is a single value assigned to more than one interface.Typically,these interfaces belong to different devices.A packet sent to an anycast address is routed to only onedevice.It is sent to the "nearest" interface having that address as defined by the routing protocol'smeasure of distance.For example, a World Wide Web (WWW) site may be mirrored on several servers.By assigning an anycast address to these servers, requests for connectivity to that WWW site areautomatically routed to only one server: the server nearest the user.Note In a routed environment, the "nearest" interface might not be the one that is physically closest.Routers can use a surprising array of metrics to calculate routes.Identifying the nearest one depends onthe actual routing protocol used, as well as that protocol's metrics.This topic is addressed in detail in PartIII, "Routing Protocols."Anycast addresses are formed from the unicast address space and may take the form of any unicastaddress type.Anycast addresses are formed by just assigning the same unicast address to more than oneinterface.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2905.htm (14 of 15) [02/02/2001 11.37.21] Internet Protocols VersionsIPv6 Multicast Address StructuresMulticasting was supported in IPv4, but required the use of obscure Class D addressing.IPv6 eliminatesClass D addresses in favor of a new address format that permits trillions of possible multicast groupcodes.Each group code identifies two or more packet recipients.The scope of a particular multicastaddress is flexible.Each address can be confined to a single system, restricted within a specific site,associated with a particular network link, or distributed globally.It should be noted that IP broadcasts have also been eliminated in favor of the new multicasting addressformat.Despite the potential benefits of IPv6, the migration from IPv4 is not risk free.The extension of theaddress length from 32 to 128 bits automatically limits interoperability between IPv4 and IPv6.IPv4-only nodes cannot interoperate with IPv6-only nodes because the address architectures are notforward-compatible.This business risk, in combination with the ongoing evolution of IPv4, will likelycontinue to forestall the acceptance of IPv6 in the marketplace.SummaryThe two versions of routed protocols described throughout this chapter, IPv4 and IPv6, are remarkablydissimilar in many respects.Together, they represent the past, present, and future of routing.Understanding these routed protocols, particularly their addressing and subnetting schemes, will help youunderstand how routers actually forward data from source to destination machines.As you examine the other aspects of internetworking, it will become increasingly apparent why some ofthe header fields are necessary, as well as how they are used.Posted: Tue Mar 2 15:40:12 PST 1999Copyright 1989-1999©Cisco Systems Inc.Copyright © 1997 Macmillan Publishing USA, a Simon & Schuster Companyhttp://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2905.htm (15 of 15) [02/02/2001 11.37.21] Transmission TechnologiesTable of ContentsTransmission TechnologiesLAN TechnologiesEthernet10 Mbps Ethernet MDIs100 Mbps Ethernet MDIsGigabit Ethernet MDIsToken RingToken Ring Transmission MediaFDDIFDDI Transmission MediaFDDI Connection TypesWraparoundsATMWAN TechnologiesLeased LinesCircuit-Switched FacilitiesIntegrated Services Digital NetworkSwitched 56Packet-Switched FacilitiesX
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