31 Days Before Your CCNA Exam

140 31 Days Before Your CCNA ExamWhen abbreviated, this would be2000:1234:5678:9ABC::/64If the prefix length does not fall on a quartet boundary (is not a multiple of 16), the prefix valueshould list all the values in the last quartet. For example, assume the prefix length in the previousexample is /56. So, by convention, the rest of the fourth quartet should be written, after being setto binary 0s, as follows:2000:1234:5678:9A00::/56The following list summarizes some key points about how to write IPv6 prefixes:■The prefix has the same value as the IP addresses in the group for the first number of bits, asdefined by the prefix length.■ Any bits after the prefix-length number of bits are binary 0s.■■The prefix can be abbreviated with the same rules as IPv6 addresses.If the prefix length is not on a quartet boundary, write down the value for the entire quartet.Table 19-2 shows several sample prefixes, their format, and a brief explanation.Table 19-2Example IPv6 Prefixes and Their MeaningsPrefix Explanation Incorrect Alternative2000::/3 All addresses whose first 3 bits are equal to the 2000/3 (omits ::)first 3 bits of hex number 2000 (bits are 001) 2::/3 (omits the rest of the first quartet)2340:1140::/26 All addresses whose first 26 bits match 2340:114::/26 (omits the lastthe listed hex numberdigit in the second quartet)2340:1111::/32 All addresses whose first 32 bits match 2340:1111/32 (omits ::)the listed hex numberIPv6 Global Unicast AddressIPv6 has an address format that enables aggregation upward eventually to the ISP. An IPv6 globalunicast address is globally unique. Similar to a public IPv4 address, it can be routed in the Internetwithout any modification. An IPv6 global unicast address consists of a 48-bit global routing prefixand a 16-bit subnet ID, as shown in Figure 19-1.Figure 19-1Global Unicast AddressGlobal Routing Prefix/23 /3216-BitSubnet/48Field/6420010D B8Interface IDRegistryISP PrefixSite PrefixSubnet Prefix