In computer networking, addressing mode refers to the mechanism how
we address a host on the network. IPv6 offers several types of modes by
which a single host can be addressed, more than one host can be
addressed at once or the host at closest distance can be addressed.
Unicast
In unicast mode of addressing, an IPv6 interface (host) is uniquely
identified in a network segment. The IPv6 packet contains both source
and destination IP addresses. A host interface is equipped with an IP
address which is unique in that network segment. A network switch or
router when receives a unicast IP packet, destined to single host, sends
out to one of its outgoing interface which connects to that particular
host. [Image: Unicast Messaging]
Multicast
The IPv6 multicast mode is same as that of IPv4. The packet destined
to multiple hosts is sent on a special multicast address. All hosts
interested in that multicast information, need to join that multicast
group first. All interfaces which have joined the group receive the
multicast packet and process it, while other hosts not interested in
multicast packets ignore the multicast information. [Image: Multicast Messaging]
Anycast
IPv6 has introduced a new type of addressing, which is called Anycast
addressing. In this addressing mode, multiple interfaces (hosts) are
assigned same Anycast IP address. When a host wishes to communicate
with a host equipped with an Anycast IP address, sends a Unicast
message. With the help of complex routing mechanism, that Unicast
message is delivered to the host closest to the Sender, in terms of
Routing cost. [Image: Anycast Messaging]Let’s take an example of TutorialPoints.com Web Servers, located in
all continents. Assume that all Web Servers are assigned single IPv6
Anycast IP Address. Now when a user from Europe wants to reach
TutorialsPoint.com the DNS points to the server which is physically
located in Europe itself. If a user from India tries to reach
Tutorialspoint.com, the DNS will then point to Web Server physically
located in Asia only. Nearest or Closest terms are used in terms of
Routing Cost.
In the above picture, When a client computer tries to reach a Server,
the request is forwarded to the Server with lowest Routing Cost.
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