128 Understanding IPv6,Second Edition Prefix Information Option The Prefix Information option is sent in Router Advertisement messages to indicate both address prefixes and information about address autoconfiguration.There can be multiple Pre- fix Information options included in a Router Advertisement message,indicating multiple address prefixes. Figure 6-6 shows the structure of the Prefix Information option. Type =3 Length =4 Prefix Length On-Link Flag Autonomous Flag Router Address Flag Site Prefix Flag Reserved 1 Valid Lifetime Preferred Lifetime Reserved 2 Site Prefix Length Prefix wwwwwwww wwwwwwww Figure 6-6 The structure of the Prefix Information option The fields in the Prefix Information option are as follows: Type The value of this field is 3. Length The value of this field is 4.(The entire option is 32 bytes in length.) Prefix Length The Prefix Length field indicates the number of leading bits in the Prefix field that make up the address prefix.The size of this field is 8 bits.The Prefix Length field has a value from 0 through 128.Because typical prefixes advertised are for subnet identifiers,the Prefix Length field is usually set to 64. On-Link flag The On-Link flag indicates,when set to 1,that the addresses implied by the included prefix are available on the link on which this Router Advertisement mes- sage was received.When this flag is set to 0,it is not assumed that the addresses that match the prefix are available on-link.The size of this field is I bit. Autonomous flag The Autonomous flag indicates,when set to 1,that the included pre- fix is used to create an autonomous(or stateless)address configuration.When this flag is set to 0,the included prefix is not used to create a stateless address configuration.The size of this field is 1 bit Router Address flag The Router Address flag is defined in RFC3775 for Mobile IPv6. For more information,see Appendix F.￾         !"#$%&"'!()*#'&'+*#'&#,,&*#&-'.*!/01!*#,(&*(,,)2,*'#&0#3)*4'* )00!,,+!"#$,)&0#&"'!()*#'&)4'.*)00!,,).*'3'&"#2.!)*#'&5!3)&4(.6*#+6 !7 "#$%&"'!()*#'&'+*#'&,#&36.00#&)-'.*!/01!*#,(&*(,,)28#&0#3)*#&2(.6*#+6 )00!,,+!"#$,5 9#2.!:7:,';,*,*!.3*.!'"* !"#$%&"'!()*#'&'+*#'&5 < => ?@ABCDEFCEDAGHC@AIDAHJKLMHGDNOCJGMGPCJGM "#60,#&* !"#$%&"'!()*#'&'+*#'&)!),"'66';,Q R ST 1)6.'"*#,"#60#,U5 R V W 1)6.'"*#,"#60#,X5Y&*#!'+*#'&#,UZ4[*,#&6&2*5\ R V W  !"#$]&2*"#60#&0#3)*,*&.(4!'"6)0#&24#*,#&* !"#$ "#60*)*()^.+*)00!,,+!"#$5,#_'"*#,"#60#,`4#*,5 !"#$]&2* "#60),)1)6."!'(a*!'.2bZ`5c3).,*[+#3)6+!"#$,)01!*#,0)!"'!,.4&* #0&*#"#!,8* !"#$]&2*"#60#,.,.)66[,**':X5 R >Vde f&7]#&^"6)2#&0#3)*,8;&,**'b8*)**)00!,,,#(+6#04[ *#&36.00+!"#$)!)1)#6)46'&*6#&^'&;#3*#,-'.*!/01!*#,(&*(,7 ,)2;),!3#105g&*#,"6)2#,,**'a8#*#,&'*),,.(0*)**)00!,,,*)* ()*3*+!"#$)!)1)#6)46'&76#&^5,#_'"*#,"#60#,b4#*5 R h= = e /.*'&'('.,"6)2#&0#3)*,8;&,**'b8*)**#&36.00+!7 "#$#,.,0*'3!)*)&).*'&'('.,Y'!,*)*6,,\)00!,,3'&"#2.!)*#'&5g&*#,"6)2 #,,**'a8*#&36.00+!"#$#,&'*.,0*'3!)*),*)*6,,)00!,,3'&"#2.!)*#'&5 ,#_'"*#,"#60#,b4#*5 R i= h e -'.*!/00!,,"6)2#,0"#&0#&-9jUkkl"'!m'4#6% 1:5 9'!('!#&"'!()*#'&8,/++&0#$95 nopq rqstuv wx y z{q|}~rqstuv s€r}s‚ƒ„t …†u‡s‡ˆ‡†‰‚ƒ„t Š‡†uq{…‹‹{q‰‰‚ƒ„t Œ}uqz{q|}~‚ƒ„t Šq‰q{q‹Ž „ƒ}‹r}|qu}ˆq z{q|q{{q‹r}|qu}ˆq Šq‰q{q‹ z{q|}~ Œ}uqz{q|}~rqstuv