What are Unicast, Multicast, and Anycast Addresses?

IPv6 Unicast Addresses are used for one-to-one communication; currently there
are 3 types of Unicast addresses; Global, Unique-local, and Link-Local.



Global Unicast Addresses or GUA’s are used by devices for one-to-one
communication across the IPv6 Internet; and every GUA belongs to a defined
Global Scope. GUA’s are easy to identify because their values are always 2000 or
higher; meaning the first three high order bits of every GUA that is created,
equals “001” or 2000::/3. A GUA is made up of three parts; the Global Routing
Prefix, the Subnet Identifier, and the Interface Identifier.



Unique-Local Addresses or ULA’s are used by devices for one-to-one
communication within an organization (site); and all ULA’s that are used within
an organization (site) belongs to the same Unique-Local Scope. ULA’s are easily
identifiable because the value of a ULA begins with FD00::/8, which simply
means that the first eight high order bits of every ULA address are equal to
“11111101”. A ULA is made up of three parts; the Global Identifier, Subnet
Identifier, and the Interface Identifier.



Link-Local Addresses are used by devices for one-to-one communication within a
layer 2 domain, in other words, link-local addresses are used by devices for one-
to-one communication within a router’s boundary (the local link). Link-Local
Addresses are easily identifiable because the value of a link-local address begins
with FE80::/10, which simply means that the first 10 high order bits of every link-
local address are equal to “1111111010” and the remaining 54 high order bits are
equal to zero. Now, just in case you were wondering, the 64 lower bits of a link-
local address are used for the Interface Identifier.



IPv6 Multicast addresses are used for one-to-many communication; meaning a
multicast address, identifies a group of network interfaces (devices) and when a
packet of data is sent to a multicast address, that packet is sent to all of the
network interfaces (devices) that are in the multicast group.



Multicast addresses are easily identifiable because the value of a IPv6 multicast
address begins with “FF” (FF00::/8), which simply means that the first 8 high
order bits are equal to one or “11111111”. A IPv6 multicast address, also has a 4
bit Flag field which is used to inform if the multicast address is a well-known
address (which is a multicast address that was given to you by your ISP) or a not
well-known multicast address (which is multicast address that was locally
generated). If the multicast address is a well-known address then the value of all
four bits in the Flag field will be equal to zero. A IP6 multicast address, also has a
4 bit Scope field, which is used to tell the type of Scope that a IPv6 mulicast
address belongs to. A IPv6 multicast address can belong to either one of the
following Scopes:



Decimal value Binary Value Address Scope

1 0001 Interface / Node-local Scope

2 0010 Link local Scope

3 0011 Subnet-local Scope

4 0100 Admin-local Scope

5 0101 Site-local Scope

8 1000 Organization-local Scope

E 1110 Global-local Scope

IPv6 multicast addresses are better to use than IPv4 multicast addresses, because
the address range for IPv6 multicasting is much bigger than IPv4’s Class D range.

IPv6 Anycast addresses are used for one-to-nearest communication, meaning an
Anycast address is used by a device to send data to one specific recipient
(interface) that is the closest out of a group of recipients (interfaces). You would
normally want to use Anycast addresses for Load Balancing. Think about it for a
minute. Let’s say you need to send a user’s request to one of many devices
(interfaces); and you don’t really care which of the designated devices handles the
request, as long as the request is taken care of. By using Anycast addresses, each
request is automatically sent to the device (interface) that is in the closest
geographic proximity to the computer that is making the request. In certain other
situations, Anycast addresses can even be used to provide fault tolerance should a
router fail. The failure can be detected, and requests can be redirected to the next
closest router. Currently, IPv6 Anycast addresses have no special addressing
scheme; they are considered to be structurally indistinguishable from unicast
addresses, so that means nodes have to be configured to understand that the
unicast address that has been assigned to their interfaces is an Anycast address.

To your success,



Charles Ross



CCNP #CSCO10444244