52 Understanding IPv6,Second Edition How Many Blocks or Bits in : To determine how many 0 blocks are represented by the::,you can count the number of blocks in the compressed address and subtract this number from 8.To determine how many 0 bits are represented by the::,multiply the number of blocks the::represents by 16.For example,in the address FF02:2,there are two blocks(the "FF02"block and the "2"block).The number of blocks expressed by the :is 6(8-2).The number of bits expressed by the::is 96(96=6x 16).Zero compression can be used only once in a given address.Otherwise,you could not determine the number of0 blocks or bits represented by each instance of :: IPv6 Prefixes The prefix is the part of the address where the bits have fixed values or are the bits that define a route or subnet.Prefixes for IPv6 subnets and summarized routes are expressed in the same way as Classless Inter-Domain Routing (CIDR)notation for IPv4.An IPv6 prefix is written in address/prefix-length notation. For example,2001:DB8:2A0:2F3B::/64 is a subnet prefix and 2001:DB8:3F::/48 is a summa- rized route prefix.As described earlier in this chapter,the 64-bit prefix is used for individual subnets to which nodes are attached.All subnets have a 64-bit prefix.Any prefix that is less than 64 bits is a summarized route or an address range that is summarizing a portion of the IPv6 address space. Note IPv4 implementations commonly use a dotted-decimal representation of the prefix length known as the subnet mask.A subnet mask is not used for IPv6.Only the prefix length notation is supported. An IPv6 prefix is relevant only for routes or address ranges,not for individual unicast addresses.In IPv4,it is common to express an IPv4 address with its prefix length.For exam- ple,192.168.29.7/24 (equivalent to 192.168.29.7 with the subnet mask 255.255.255.0) denotes the IPv4 address 192.168.29.7 with a 24-bit subnet mask.Because IPv4 addresses are no longer class-based,you cannot assume the class-based subnet mask based on the value of the leading octet.The prefix length is included so that you can determine which bits identify the subnet and which bits identify the host on the subnet.Because the number of bits used to identify the subnet in IPv4 is variable,the prefix length is needed to separate the subnet prefix from the host ID. In common IPv6 practice,however,there is no notion of a variable-length subnet prefix.At the individual IPv6 subnet level for currently defined unicast IPv6 addresses,the number of bits used to identify the subnet is always 64 and the number of bits used to identify the host on the subnet is always 64.Therefore,while unicast IPv6 addresses written with their prefix lengths￾  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`abbcd`aefghiajklm 0),3,/)0A D)- +E3.;7+= F55B<Zn@<F]5<FDon<<cC\ /: 3 :>60+, ;-+?/E 30* F55B<Zn@<oD<<c\@ /: 3 :>..3Y -/W+* -)>,+ ;-+?/EA ]: *+:8-/6+* +3-7/+- /0 ,1/: 813;,+-= ,1+ C\Y6/, ;-+?/E /: >:+* ?)- /0*/Q/*>37 :>60+,: ,) 21/81 0)*+: 3-+ 3,,381+*A ]77 :>60+,: 13Q+ 3 C\Y6/, ;-+?/EA ]04 ;-+?/E ,13, /: 7+:: ,130 C\ 6/,: /: 3 :>..3-/W+* -)>,+ )- 30 3**-+:: -30P+ ,13, /: :>..3-/W/0P 3 ;)-,/)0 )? ,1+ VUQC 3**-+:: :;38+A p  qrst uvwxyvyz{|{u}z~ }vv}zx€ ~y | ‚}{{y‚ƒ‚yuv|x „yw„y~yz{|{u}z }… {†y w„y…u‡ xyzˆ{† ‰z}Šz |~ {†y ‹ŒŽ ‘’‹ • ~zy{ v|~‰ u~ z}{ ~y‚ …}„ qrs zx€ {†y w„y…u‡ xyzˆ{† z}{|{u}z u~ ~ww}„{y‚ ]0 VUQC ;-+?/E /: -+7+Q30, )074 ?)- -)>,+: )- 3**-+:: -30P+:= 0), ?)- /0*/Q/*>37 >0/83:, 3**-+::+:A V0 VUQ\= /, /: 8)..)0 ,) +E;-+:: 30 VUQ\ 3**-+:: 2/,1 /,: ;-+?/E 7+0P,1A D)- +E3.Y ;7+= BLFABC@AFLA™cF\ G+š>/Q37+0, ,) BLFABC@AFLA™ 2/,1 ,1+ :>60+, .3:9 F››AF››AF››A5J *+0),+: ,1+ VUQ\ 3**-+:: BLFABC@AFLA™ 2/,1 3 F\Y6/, :>60+, .3:9A n+83>:+ VUQ\ 3**-+::+: 3-+ 0) 7)0P+- 873::Y63:+*= 4)> 8300), 3::>.+ ,1+ 873::Y63:+* :>60+, .3:9 63:+* )0 ,1+ Q37>+ )? ,1+ 7+3*/0P )8,+,A (1+ ;-+?/E 7+0P,1 /: /087>*+* :) ,13, 4)> 830 *+,+-./0+ 21/81 6/,: /*+0,/?4 ,1+ :>60+, 30* 21/81 6/,: /*+0,/?4 ,1+ 1):, )0 ,1+ :>60+,A n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