Networks X

Three major types of IPv6 addresses:


• Global unicast
• Unique local
• Link local



Global Unicast Addresses

Global unicast IPv6 addresses function in a manner similar to public
IPv4 addresses. Typically, an organization that wishes to deploy IPv6
will be assigned a registered, unique global routing prefix (which is
equivalent to an IPv4 network address). Every global routing prefix
must be unique throughout the entire Internet. This ensures that every
IPv6 address assigned to individual net work hosts is globally unique
as well.



The global routing prefix is assigned by your ISP and is usually 48
bits long (/48). This allows you to use the remaining 16 bits in the
global routing prefix to create your own internal subnets. Because of
the size of the prefix (/64), you can define up to 216 subnets. That’s
a huge number! All subnets within the same organization will use the
same global routing prefix that was assigned by the ISP, but will use
a different value in the subnet portion of the prefix.



When deploying IPv6, you don’t actually have to use global unicast
addresses. If appropriate, you could implement unique local addressing
instead.






Unique Local Addresses

Unique local addresses are similar to private IPv4 addresses and are
normally used for networks that are not connected to a public network
(such as the Internet). Unique local addresses are not registered, so
they cannot be used on a public network without network address
translation.


The first eight bits of unique local addresses use a prefix that
always begins with FC or FD. The next 40 bits are used for the global
ID, which is a randomly generated number that is intended to create a
high probability of uniqueness. Therefore, unique local addresses are
actually designed to be globally unique, even though they can’t be
used on the Internet. The remaining 16 bits in the prefix after the
Global ID are used for creating subnets, just as we saw with globally
unique addressing. The rest of the address is used for the interface
IDs that are assigned to individual network hosts.


Before we end, we also need to look at link-local addresses.





Link-Local Addresses

IPv6 also uses special unicast addresses called link-local addresses
that are used just for communications within a local network segment.
Routers never forward packets addressed to link-local addresses on
other subnets. As such, link-local addresses cannot be used for
communications between hosts on different network segments. Link-local
addresses are used for specific purposes, including the following:



• Automatic address configuration
• Neighbor discovery
• Routerless subnets



The prefix used by link-local addresses begins with FE8, FE9, FEA, or
FEB. When working with IPv6, you will notice that every IPv6 host has
at least one link-local address that is automatically assigned when
the system comes up.











LX0-104 Exam Objectives (R)