Data Communications Networking Devices - 4th Ed.pdf

230 __________________________________________________________ WIDE AREA NETWORKSgroup of interfaces. Thus, a packet transmitted to a unicast address is sent to aspeci®c location while a packet sent to a multicast address are delivered to eachmember of the multicast group. The third type of IPv6 address, anycast, identi®esa set of interfaces similar to multicast address; however, instead of being deliveredto all members of the group, packets sent to an anycast address are delivered toonly one interfaceÐthe nearest member of the anycast group.Through the use of an anycast address a host does not have to know the speci®caddress of a router since a group of routers could be assigned membership in ananycast group. Then, the host can be con®gured to use an anycast group address asits gateway address. This address would not require modi®cation if at a later datethe network was restructured to include rearranging the location of routers. This isbecause packets would continue to be sent to the nearest member of the anycastgroup. Based upon the use of anycast addresses, some of the recon®gurationproblems associated with network restructuring now commonly experienced underIPv4 may be minimized under IPv6.The format pre®xIPv6 allocates address space similar to the manner by which IPv4 addresses aregrouped into classes based upon the composition of the ®rst few bits of the address.This method of address space allocation is accomplished by a variable length ®eldof unique bits denoted by the term Format Pre®x. Table 2.7 lists the initialallocation of IPv6 address space to include the allocation assignment, binary pre®x,and the fraction of the 128-bit address space assigned to the noted allocation.Address notations and examplesUnder the IPv6 design 128-bit addresses are written as eight 16-bit integersseparated by colons. Each integer is represented by four hex digits, resulting in areasonable expectation that you will have to enter 32 hex digits to specify an IPaddress. To facilitate the entry of 128-bit addresses, you can skip leading zeros ineach hexadecimal integer sequence as well as use a double colon : :) inside anaddress as a mechanism to replace a set of consecutive null 16-bit numbers. Bothfeatures can be expected to be used frequently during the initial stage of IPv6deployment since only a portion of all 128 bits of the address need to used. Thiswill more than likely result in addresses containing many zeros, enabling the abilityto skip leading zeros in each hexadecimal component and use double colons toreplace consecutive null numbers to facilitate the IP con®guration process. Forexample, during what will probably be a lengthy transition period as the Internetmoves from IPv4 to IPv6, IPv4-compatible IPv6 addresses will consist of a 32-bitIPv4 address in the low-ordered 32 bits of the IPv6 address space. Thus, suchaddresses transported in an IPv6 format will be pre®xed with 96 zeros.To illustrate the compaction of IPv6 addresses, consider the following 128-bitaddress created for illustrative purpose only:504A : 0000 : 0000 : 0000 : 00FC : ABCD : 3A1F : 4D3A