IPV4 to IPV6

IPv6 Mobility

Tukiyooo 0 IPv6, mobile phone, Setup IPv6 01:23

When a host is connected to one link or network, it acquires an IP 
address and all communication happens using that IP address on that 
link.  As soon as, the same host changes its physical location, that is,
 moves into some different area / subnet / network / link, its IP 
address changes accordingly and all communication happening on the host 
using old IP address, goes down.<br />
IPv6 mobility provides a mechanism which equips a host with an 
ability to roam around among different links without losing any 
communication/connection and its IP address.<br />
Multiple entities are involved in this technology:<br />
<ul class="list">
<li><b>Mobile Node</b>:  The device which needs IPv6 mobility. </li>
<li><b>Home Link</b>: This link is configured with the home subnet prefix and this is where the Mobile IPv6 device gets its Home Address.</li>
<li><b>Home Address</b>: This is the address which Mobile Node 
acquires from Home Link.  This is permanent address of Mobile Node.  If 
the Mobile Node remains in the same Home Link, the communication among 
various entities happens as usual.</li>
<li><b>Home Agent</b>: This is a router which acts as registrar for 
Mobile Nodes. Home Agent is connected to Home Link and maintains 
information about all Mobile Nodes, their Home Addresses and their 
present IP addresses.</li>
<li><b>Foreign Link</b>: Any other Link which is not Mobile Node’s Home Link.</li>
<li><b>Care-of Address</b>: When a Mobile Node attaches to a Foreign 
Link, it acquires a new IP address of that Foreign Link’s subnet.  Home 
Agent maintains the information of both Home Address and Care-of 
Address.  Multiple Care-of addresses can be assigned to Mobile Node, but
 at any instance only one Care-of Address has binding with Home Address.</li>
<li><b>Correspondent Node</b>: Any IPv6 enable device which intends to have communication with Mobile Node.</li>
</ul>
<h2>
Mobility Operation</h2>
When Mobile Node stays in its Home Link, all communications happen on its Home Address.  As shown below:<br />
<figure>
<img caption="Mobile Node in Home Link" src="http://www.tutorialspoint.com/ipv6/images/MN_in_HL.jpg" />
<figcaption align="center">[<i>Image: Mobile Node connected to Home Link</i>]</figcaption>
</figure>When Mobile Node leaves its Home Link and is connected to some 
Foreign Link, the Mobility feature of IPv6 comes into play.  After 
connecting to Foreign Link, Mobile Node acquires an IPv6 address from 
Foreign Link.  This address is called Care-of Address.  Mobile Node 
sends binding request to its Home Agent with the new Care-of Address.  
Home Agent binds Mobile Node’s Home Address with Care-of Address, 
establishing a Tunnel between both.<br />
Whenever a Correspondent Node tries to establish connection with 
Mobile Node (on its Home Address), the Home Agent intercepts the packet 
and forwards to Mobile Node’s Care-of Address over the Tunnel which was 
already established.<br />
<figure>
<img caption="Mobile Node in Foreign Link" src="http://www.tutorialspoint.com/ipv6/images/MN_in_FL.jpg" /> 
<figcaption align="center">[<i>Image: Mobile Node connected to Foreign Link</i>]</figcaption>
</figure><h2>
Route Optimization</h2>
<br />
When a Correspondent Node initiate communication by sending packets 
to Mobile Node on Home Address, these packets are tunneled to Mobile 
Node by Home Agent.  In Route Optimization mode, when the Mobile Node 
receives packet from Correspondent Node, it does not forward replies to 
Home Agent.  Rather it sends its packet directly to Correspondent Node 
using Home Address as Source Address.  This mode is optional and not 
used by default.<br />
<span style="background-color: #edf4ff; color: #888888; font-family: Arial, Helvetica, sans-serif; font-size: 13px;"><a href="http://ipv4-ipv6.blogspot.com/2014/07/when-host-is-connected-to-one-link-or.html">http://ipv4-ipv6.blogspot.com/2014/07/when-host-is-connected-to-one-link-or.html</a></span>

IPv6 Mobility

When a host is connected to one link or network, it acquires an IP address and all communication happens using that IP address on that lin...

IPv6 Subnetting

Tukiyooo 0 IPv6, Setup IPv6, Subnetting 01:21

<a href="http://ipv4-ipv6.blogspot.com/" target="_blank">In IPv4</a>, addresses were created in classes.  Classful IPv4 addresses 
clearly defines the bits used for network prefixes and the bits used for
 hosts on that network.  To subnet in IPv4 we play with the default 
classful netmask which allows us to borrow hosts bit to be used as 
subnet bits.  This results in multiple subnets but less hosts per 
subnet.  That is, when we borrow host bit to create a subnet that costs 
us in lesser bit to be used for host addresses.<br />
IPv6 addresses uses 128 bits to represent an address which includes 
bits to be used for subnetting.  Second half of the address (least 
significant 64 bits) is always used for Hosts only.  Therefore, there is
 no compromise if we subnet the network.<br />
<figure>
<img caption="IPv6 Subnetting" src="http://www.tutorialspoint.com/ipv6/images/subnetting.jpg" />
<figcaption align="center">[<i>Image: IPv6 Subnetting</i>]</figcaption>
</figure>16 Bits of subnet is equivalent to IPv4’s Class B Network.  Using 
these subnet bits an organization can have more 65 thousands of subnets 
which is by far, more than enough.<br />
Thus routing prefix is /64 and host portion is 64 bits.  We though, 
can further subnet the network beyond 16 bits of Subnet ID, borrowing 
hosts bit but it is recommended that 64 bits should always be used for 
hosts addresses because auto-configuration requires 64 bits.<br />
IPv6 subnetting works on the same concept as Variable Length Subnet Masking in IPv4.<br />

<br />
/48 prefix can be allocated to an organization providing it the 
benefit of having up to /64 subnet prefixes, which is 65535 
sub-networks, each having 264 hosts.  A /64 prefix can be assigned to a 
point-to-point connection where there are only two hosts (or IPv6 
enabled devices) on a link.

IPv6 Subnetting

In IPv4 , addresses were created in classes. Classful IPv4 addresses clearly defines the bits used for network prefixes and the bits used ...

IPv6 Summary

Tukiyooo 0 IPv6, Setup IPv6, Summary 01:20

<a href="http://ipv4-ipv6.blogspot.com/" target="_blank">IPv4 since 1982</a>, has been an undisputed leader of Internet.  With 
IPv4’s address space exhaustion IPv6 is now taking over the control of 
Internet, which is called Internet2.<br />
IPv4 is widely deployed and migration to IPv6 would not be easy.  So 
far IPv6 could penetrate IPv4’s address space by less than 1%.  <br />
The world has celebrated ‘World IPv6 Day’ on June 08, 2011 with a 
purpose to test IPv6 address over Internet in full.  On June 06, 2012 
the Internet community officially launched IPv6.  This day all ISPs who 
were offering IPv6 were to enable it on public domain and were to keep 
it enable.  All the device manufacturer also participated to offer IPv6 
by-default enabled on devices.<br />
This was a step towards encouraging Internet community to migrate to IPv6.<br />
Organizations are provided plenty of ways to migrate from IPv4 to 
IPv6.  Also organization, willing to test IPv6 before migrating 
completely can run both IPv4 and IPv6 simultaneously.  Networks of 
different IP versions can communicate and user data can be tunneled to 
walk to the other side.<br />
<h2>
Future of IPv6</h2>
IPv6 enabled Internet version 2 will replace todays IPv4 enabled 
Internet.  When Internet was launched with IPv4, developed countries 
like US and Europe took the larger space of IPv4 for deployment of 
Internet in their respective countries keeping future need in mind.  But
 Internet exploded everywhere reaching and connecting every country of 
the world increasing the requirement of IPv4 address space.  As a 
result, till this day US and Europe have many IPv4 address space left 
with them and countries like India and China are bound to address their 
IP space requirement by means of deployment of IPv6.<br />
Most of the IPv6 deployment is being done outside US, Europe.  India 
and China are moving forward to change their entire space to IPv6.  
China has announced a five year deployment plan named China Next 
Generation Internet.<br />
After June 06, 2012 all major ISPs were shifted to IPv6 and rest of them are still moving.<br />

<br />
IPv6 provides ample of address space and is designed to expand 
today’s Internet services.  Feature-rich IPv6 enabled Internet version 2
 may deliver more than expected.<br />
<br />
<div class="GCUXF0KCCXB" style="background-color: #edf4ff; border: 0px; color: #888888; cursor: default; font-family: Arial, Helvetica, sans-serif; font-size: 13px; margin: 0px; padding: 0px; vertical-align: baseline; word-wrap: break-word;">
<a href="http://ipv4-ipv6.blogspot.com/2014/07/ipv6-summary.html">http://ipv4-ipv6.blogspot.com/2014/07/ipv6-summary.html</a></div>
<div>
<br /></div>

IPv6 Summary

IPv4 since 1982 , has been an undisputed leader of Internet. With IPv4’s address space exhaustion IPv6 is now taking over the control of ...

IPv6 Routing

Tukiyooo 0 IPv6, IPv6 Routing, Routing, Setup IPv6 01:18

Routing concepts remain same in case of <a href="http://ipv4-ipv6.blogspot.com/" target="_blank">IPv6</a> but almost all routing 
protocol have been redefined accordingly.  We have seen in Communication
 in IPv6 segment, how a host speaks to its gateway.  Routing is a 
process to forward routable data choosing best route among several 
available routes or path to the destination.  A router is a device which
 forwards data which is not explicitly destined to it.<br />
There exists two forms of routing protocols<br />
<ul class="list">
<li><b>Distance Vector Routing Protocol</b>: A 
router running distance vector protocol advertises its connected routes 
and learns new routes from its neighbors.  The routing cost to reach a 
destination is calculated by means of hops between the source and 
destination.  A Router generally relies on its neighbor for best path 
selection, also known as “routing-by-rumors”.  RIP and BGP are Distance 
Vector Protocols.</li>
<li><b>Link-State Routing Protocol</b>: This protocol acknowledges 
the state of a Link and advertises to its neighbors.  Information about 
new links is learnt from peer routers.  After all the routing 
information has been converged, Link-State Routing Protocol uses its own
 algorithm to calculate best path to all available links.  OSPF and 
IS-IS are link state routing protocols and both uses Djikstra’s Shortest
 Path First algorithm.</li>
</ul>
Routing protocols can be divided in two categories:<br />
<ul class="list">
<li><b>Interior Routing Protocol</b>:  Protocols in 
this categories are used within an Autonomous System or organization to 
distribute routes among all routers inside its boundary.  Examples: RIP,
 OSPF.</li>
<li><b>Exterior Routing Protocol</b>:  Whereas an Exterior Routing 
Protocol distributes routing information between two different 
Autonomous Systems or organization.  Examples: BGP.</li>
</ul>
<h2>
Routing protocols</h2>
<ul class="list">
<li><b>RIPng</b><br />

RIPng stands for Routing Information Protocol Next Generation.  This 
is an Interior Routing Protocol and is a Distance Vector Protocol.  
RIPng has been upgraded to support IPv6.<br />

</li>
<li><b>OSPFv3</b></li>
Open Shortest Path First version 3 is an Interior Routing Protocol 
which is modified to support IPv6.  This is a Link-State Protocol and 
uses Djikrasta’s Shortest Path First algorithm to calculate best path to
 all destinations.<br />
<li><b>BGPv4</b><br />

BGP stands for Border Gateway Protocol.  It is the only open standard
 Exterior Gateway Protocol available.  BGP is a Distance Vector protocol
 which takes Autonomous System as calculation metric, instead of number 
of routers as Hop.  BGPv4 is an upgrade of BGP to support IPv6 routing.</li>
</ul>
<h2>
Protocols changed to support IPv6:</h2>
<br />
<ul class="list">
<li><b>ICMPv6</b>: Internet Control Message Protocol
 version 6 is an upgraded implementation of ICMP to accommodate IPv6 
requirements.  This protocol is used for diagnostic functions, error and
 information message, statistical purposes.  ICMPv6’s Neighbor Discovery
 Protocol replaces ARP and helps discover neighbor and routers on the 
link.</li>
<li><b>DHCPv6</b>: Dynamic Host Configuration Protocol version 6 is 
an implementation of DHCP.  Though IPv6 enabled hosts do not require any
 DHCPv6 Server to acquire IP address as they can be auto-configured.  
Neither do they need DHCPv6 to locate DNS server because DNS can be 
discovered and configured via ICMPv6 Neighbor Discovery Protocol.  Yet 
DHCPv6 Server can be used to provide these information. </li>
<li><b>DNS</b>: There has been no new version of DNS but it is now 
equipped with extensions to provide support for querying IPv6 addresses.
  A new AAAA (quad-A) record has been added to reply IPv6 query 
messages.  Now DNS can reply with both IP versions (4 & 6) without 
any change in query format.</li>
</ul>
<div>
<span style="background-color: #edf4ff; color: #888888; font-family: Arial, Helvetica, sans-serif; font-size: 13px;"><a href="http://ipv4-ipv6.blogspot.com/2014/07/ipv6-routing.html">http://ipv4-ipv6.blogspot.com/2014/07/ipv6-routing.html</a></span></div>
<div>
<br /></div>

IPv6 Routing

Routing concepts remain same in case of IPv6 but almost all routing protocol have been redefined accordingly. We have seen in Communicati...

Address Types & Formats IPv6

Tukiyooo 0 address, Addressing, Hexadecimal, Infrastructures, IP address, IPv6, Setup IPv6 01:17

<h2>
Hexadecimal Number System</h2>
Before introducing <a href="http://ipv4-ipv6.blogspot.com/" target="_blank">IPv6 Address format</a>, we shall look into 
Hexadecimal Number System.  Hexadecimal is positional number system 
which uses radix (base) of 16.  To represent the values in readable 
format, this system uses 0-9 symbols to represent values from zero to 
nine and A-F symbol to represent values from ten to fifteen.  Every 
digit in Hexadecimal can represent values from 0 to 15.<br />
<figure>
<img caption="Hexadecimal Conversion Table" src="http://www.tutorialspoint.com/ipv6/images/Hexadecimal_conversion_table.jpg" />
<figcaption align="center">[<i>Image: Conversion Table</i>]</figcaption>
</figure><h2>
Address Structure</h2>
An IPv6 address is made of 128 bits divided into eight 16-bits 
blocks.  Each block is then converted into 4-digit Hexadecimal numbers 
separated by colon symbol.<br />

For example, the below is 128 bit IPv6 address represented in binary format and divided into eight 16-bits blocks:<br />
<blockquote>
0010000000000001 0000000000000000 0011001000110100 1101111111100001 
0000000001100011 0000000000000000 0000000000000000 1111111011111011</blockquote>
Each block is then converted into Hexadecimal and separated by ‘:’ symbol:<br />
<blockquote>
2001:0000:3238:DFE1:0063:0000:0000:FEFB</blockquote>
Even after converting into Hexadecimal format, IPv6 address remains 
long.  IPv6 provides some rules to shorten the address.  These rules 
are:<br />
<b>Rule:1</b> Discard leading Zero(es):<br />
In Block 5, 0063, the leading two 0s can be omitted, such as (5th block):<br />
<blockquote>
2001:0000:3238:DFE1:63:0000:0000:FEFB</blockquote>
<b>Rule:2</b> If two of more blocks contains consecutive zeroes, omit
 them all and replace with double colon sign ::, such as (6th and 7th 
block):<br />
<blockquote>
2001:0000:3238:DFE1:63::FEFB</blockquote>
Consecutive blocks of zeroes can be replaced only once by :: so if 
there are still blocks of zeroes in the address they can be shrink down 
to single zero, such as (2nd block):<br />
<blockquote>
2001:0:3238:DFE1:63::FEFB</blockquote>
<h2>
Interface ID</h2>
IPv6 has three different type of Unicast Address scheme.  The second 
half of the address (last 64 bits) is always used for Interface ID.  MAC
 address of a system is composed of 48-bits and represented in 
Hexadecimal.  MAC address is considered to be uniquely assigned 
worldwide.  Interface ID takes advantage of this uniqueness of MAC 
addresses.  A host can auto-configure its Interface ID by using IEEE’s 
Extended Unique Identifier (EUI-64) format.  First, a Host divides its 
own MAC address into two 24-bits halves.  Then 16-bit Hex value 0xFFFE 
is sandwiched into those two halves of MAC address, resulting in EUI-64 
Interface ID.<br />

<img src="http://www.tutorialspoint.com/ipv6/images/interface_id.jpg" />
<br />
<figcaption align="center">[<i>Image: EUI-64 Interface ID</i>]</figcaption><b>Conversion of EUI-64 ID into IPv6 Interface Identifier</b><br />
To convert EUI-64 ID into IPv6 Interface Identifier, the most significant 7<sup><small>th</small></sup> bit of EUI-64 ID is complemented.  For example:<br />
<figure style="margin: 0px; padding: 0px;">
<img src="http://www.tutorialspoint.com/ipv6/images/eui_to_ipv6_id.png" />
<figcaption align="center">[<i>Image: IPV6 Interface ID</i>]</figcaption>
</figure><h2>
Global Unicast Address</h2>
This address type is equivalent to IPv4’s public address.  Global 
Unicast addresses in IPv6 are globally identifiable and uniquely 
addressable. <br />
<figure style="margin: 0px; padding: 0px;">
<img caption="Global Unicast Address" src="http://www.tutorialspoint.com/ipv6/images/global_unicast_address.jpg" />
<figcaption align="center">[<i>Image: Global Unicast Address</i>]</figcaption>
</figure>Global Routing Prefix: The most significant 48-bits are designated as
 Global Routing Prefix which is assigned to specific Autonomous System. 
 Three most significant bits of Global Routing Prefix is always set to 
001.<br />
<h2>
Link-Local Address</h2>
Auto-configured IPv6 address is known as Link-Local address.  This 
address always starts with FE80.  First 16 bits of Link-Local address is
 always set to 1111 1110 1000 0000 (FE80).  Next 48-bits are set to 0, 
thus:<br />
<figure style="margin: 0px; padding: 0px;">
<img caption="Link-Local Address" src="http://www.tutorialspoint.com/ipv6/images/link_local_address.jpg" />
<figcaption align="center">[<i>Image: Link-Local Address</i>]</figcaption>
</figure>Link-Local addresses are used for communication among IPv6 hosts on a
 link (broadcast segment) only.  These addresses are not routable so a 
Router never forwards these addresses outside the link.<br />
<h2>
Unique-Local Address</h2>
This type of IPv6 address which is though globally unique, but it 
should be used in local communication.  This address has second half of 
Interface ID and first half is divided among Prefix, Local Bit, Global 
ID and Subnet ID.<br />
<figure style="margin: 0px; padding: 0px;">
<img caption="Unique-Local Address" src="http://www.tutorialspoint.com/ipv6/images/unique_local_address.jpg" />
<figcaption align="center">[<i>Image: Unique-Local Address</i>]</figcaption>
</figure>
Prefix is always set to 1111 110.  
L bit, which is set to 1 if the address is locally assigned.  So far the meaning of L bit to 0 is not defined.
Therefore, Unique Local IPv6 address always starts with ‘FD’.
<br />
<h3>
Scope of IPv6 Unicast Addresses:</h3>
<figure>
<img caption="IPv6 Unicast Address Scope" src="http://www.tutorialspoint.com/ipv6/images/address_scope.jpg" />
<figcaption align="center">[<i>Image: IPv6 Unicast Address Scope</i>]</figcaption>
</figure>

<br />
The scope of Link-local address is limited to the segment.  Unique 
Local Address are though locally global but are not routed over the 
Internet, limiting their scope to an organization’s boundary.  Global 
Unicast addresses are globally unique and recognizable.  They shall make
 the essence of Internet v2 addressing.<br />
<br />
<span style="background-color: #edf4ff; color: #888888; font-family: Arial, Helvetica, sans-serif; font-size: 13px;"><a href="http://ipv4-ipv6.blogspot.com/2014/07/address-types-formats-ipv6.html">http://ipv4-ipv6.blogspot.com/2014/07/address-types-formats-ipv6.html</a></span>

Address Types & Formats IPv6

Hexadecimal Number System Before introducing IPv6 Address format , we shall look into Hexadecimal Number System. Hexadecimal is position...

IPv6 Addressing Modes

Tukiyooo 0 Addressing, IPv6, Setup IPv6 01:15

<a href="http://ipv4-ipv6.blogspot.com/" target="_blank">In computer networking</a>, 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.<br />
<h2>
Unicast</h2>
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.<br />
<figure>
<img caption="Unicast Messaging" src="http://www.tutorialspoint.com/ipv6/images/unicast.jpg" />
<figcaption align="center">[<i>Image: Unicast Messaging</i>]</figcaption>
</figure><h2>
Multicast</h2>
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.<br />

<figure>
<img caption="Multicast Messaging" src="http://www.tutorialspoint.com/ipv6/images/multicast.jpg" />
<figcaption align="center">[<i>Image: Multicast Messaging</i>]</figcaption>
</figure><h2>
Anycast</h2>
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.<br />
<figure>
<img caption="Anycast Messaging" src="http://www.tutorialspoint.com/ipv6/images/anycast.jpg" />
<figcaption align="center">[<i>Image: Anycast Messaging</i>]</figcaption>
</figure>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.<br />

<br />
In the above picture, When a client computer tries to reach a Server,
 the request is forwarded to the Server with lowest Routing Cost.<br />
<br />
<span style="background-color: #edf4ff; color: #888888; font-family: Arial, Helvetica, sans-serif; font-size: 13px;"><a href="http://ipv4-ipv6.blogspot.com/2014/07/in-computer-networking-addressing-mode.html">http://ipv4-ipv6.blogspot.com/2014/07/in-computer-networking-addressing-mode.html</a></span>

IPv6 Addressing Modes

In computer networking , addressing mode refers to the mechanism how we address a host on the network. IPv6 offers several types of modes ...

IPv6 - Headers

Tukiyooo 0 IPv6, Setup IPv6 01:13

The wonder of IPv6 lies in its header.  IPv6 address is 4 times 
larger than IPv4 but the IPv6 header is only 2 times larger than that of
 IPv4.  IPv6 headers have one Fixed Header and zero or more Optional 
(Extension) Headers.  All necessary information which is essential for a
 router is kept in Fixed Header.  Extension Header contains optional 
information which helps routers to understand how to handle a 
packet/flow.<br />
<h2>
<a href="http://ipv4-ipv6.blogspot.com/" target="_blank">Fixed Header</a></h2>
<figure style="margin: 0px; padding: 0px;">
<img caption="IPv6 Fixed Header" src="http://www.tutorialspoint.com/ipv6/images/IPv6_header.jpg" />
<figcaption align="center">[<i>Image: IPv6 Fixed Header</i>]</figcaption>
</figure>IPv6 fixed header is 40 bytes long and contains the following information.<br />
<table class="src">
<tbody>
<tr><th style="width: 5%;">S.N.</th><th>Field & Description</th></tr>
<tr><td>1</td><td><b>Version</b> (4-bits): This represents the version of Internet Protocol, i.e. 0110.</td></tr>
<tr><td>2</td><td><b>Traffic Class</b> (8-bits): These 8 bits are 
divided into two parts.  Most significant 6 bits are used for Type of 
Service, which tells the Router what services should be provided to this
 packet.  Least significant 2 bits are used for Explicit Congestion 
Notification (ECN).</td></tr>
<tr><td>3</td><td><b>Flow Label</b> (20-bits): This label is used to 
maintain the sequential flow of the packets belonging to a 
communication.  The source labels the sequence which helps the router to
 identify that this packet belongs to a specific flow of information.  
This field helps to avoid re-ordering of data packets.  It is designed 
for streaming/real-time media.</td></tr>
<tr><td>4</td><td><b>Payload Length</b> (16-bits): This field is used
 to tell the routers how much information this packet contains in its 
payload.  Payload is composed of Extension Headers and Upper Layer data.
  With 16 bits, up to 65535 bytes can be indicated but if Extension 
Headers contain Hop-by-Hop Extension Header than payload may exceed 
65535 bytes and this field is set to 0.</td></tr>
<tr><td>5</td><td><b>Next Header</b> (8-bits): This field is used to 
indicate either the type of Extension Header, or if Extension Header is 
not present then it indicates the Upper Layer PDU.  The values for the 
type of Upper Layer PDU is same as IPv4’s.</td></tr>
<tr><td>6</td><td><b>Hop Limit</b> (8-bits):  This field is used to 
stop packet to loop in the network infinitely.  This is same as TTL in 
IPv4.  The value of Hop Limit field is decremented by 1 as it passes a 
link (router/hop).  When the field reaches 0 the packet is discarded.</td></tr>
<tr><td>7</td><td><b>Source Address</b> (128-bits):  This field indicates the address of originator of the packet.</td></tr>
<tr><td>8</td><td><b>Destination Address</b> (128-bits):  This field provides the address of intended recipient of the packet.</td></tr>
</tbody></table>
<h2>
Extension Headers</h2>
In IPv6, the Fixed Header contains only information which is 
necessary and avoiding information which is either not required or is 
rarely used.  All such information, is put between the Fixed Header and 
Upper layer header in the form of Extension Headers.  Each Extension 
Header is identified by a distinct value.<br />
When Extension Headers are used, IPv6 Fixed Header’s Next Header 
field points to the first Extension Header.  If there is one more 
Extension Header, then first Extension Header’s ‘Next-Header’ field 
point to the second one, and so on.  The last Extension Header’s 
‘Next-Header’ field point to Upper Layer Header.  Thus all headers from 
point to the next one in a linked list manner.<br />
If the Next Header field contains value 59, it indicates that there’s no header after this header, not even Upper Layer Header.<br />
The following Extension Headers must be supported as per RFC 2460:<br />

<img caption="Extension Headers" src="http://www.tutorialspoint.com/ipv6/images/extension_headers.jpg" />
<br />
The sequence of Extension Headers should be:<br />

<img caption="Extension Headers Sequence" src="http://www.tutorialspoint.com/ipv6/images/extension_headers_sequence.jpg" />
<br />
These headers:<br />
<ul class="list">
<li>1. Should be processed by First and subsequent destinations.</li>
<li>2. Should be processed by Final Destination. </li>
</ul>
Extension Headers are arranged one after another in a Linked list manner, as depicted in the diagram below:<br />

<br />
<figure style="margin: 0px; padding: 0px;">
<img caption="Extension Headers Connection" src="http://www.tutorialspoint.com/ipv6/images/extension_header_sequence.jpg" />
<figcaption align="center">[<i>Image: Extension Headers Connected Format</i>]</figcaption><figcaption align="center"><br /></figcaption><figcaption align="center"><br /></figcaption><figcaption align="center"><span style="background-color: #edf4ff; color: #888888; font-family: Arial, Helvetica, sans-serif; font-size: 13px; text-align: start;"><a href="http://ipv4-ipv6.blogspot.com/2014/07/the-wonder-of-ipv6-lies-in-its-header.html">http://ipv4-ipv6.blogspot.com/2014/07/the-wonder-of-ipv6-lies-in-its-header.html</a></span></figcaption></figure>

IPv6 - Headers

The wonder of IPv6 lies in its header. IPv6 address is 4 times larger than IPv4 but the IPv6 header is only 2 times larger than that of I...