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CompTIA A+ Certification All-in-One Exam Guide

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Figure 19-14 Link-local address in ipconfig

The second 64 bits of a link-local address, the interface ID, are generated

in two ways. Every current operating system generates a 64-bit random

number. Very old operating systems, such as Windows XP and Windows

Server 2003, used the device’s MAC address to create a 64-bit number called

an Extended Unique Identifier, 64-bit (EUI-64).

The link-local address does all the hard work in IPv6, and, as long as you

don’t need an Internet connection, it’s all you need. The old concepts of static

and DHCP addressing don’t really make much sense in IPv6 unless you have

dedicated servers (even in IPv6, servers generally still have static IP

addresses). Link-local addressing takes care of all your local network needs.

IPv6 Prefix Lengths

Systems use IPv6 prefix lengths to determine whether to send packets to a

local MAC address or to the default gateway to send the packets out to the

Internet. But you need to focus on two rules:

• The last 64 bits of an IPv6 address are generated by the NIC, leaving a

maximum of 64 bits for the prefix. Therefore, no prefix is ever longer

than /64.

• The five Regional Internet Registries (RIRs) pass out /48 prefixes to

big ISPs and end users who need large allotments. ISPs and others will

borrow another 16 bits for subnetting and then pass out /64 interface

IDs to end users. Link-local addressing uses a prefix length of /64.

Other types of IPv6 addresses get the subnet information automatically

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