IPv6

Answered Question
Sep 16th, 2008

Hi, i'm looking for some guidance regarding IPv6. This threw me loads last night. :-(

I understand the very basics regarding IPv6 and how the process of address assignment occurs with the unique local (FE), link local (FD) and how the MAC address is incorporated into the last 64 bits of the 128 bit address with the FFFE stuck between the MAC.

I know the link local address is used for the LAN communication but where does the global address come into play? Do each device on the LAN have 2 addresses?

I think it might be helpful if somebody would be kind enough to lay out a simple network with a few IPv6 addresses assigned so I can how things look.

Or an explanation would be great. 

Thanks a lot!

Garry

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Correct Answer by Giuseppe Larosa about 8 years 2 months ago

Hello Garry,

your understanding is correct:

link local : confined on a link

unique local: can be routed as used as IPv4 RFC1918 private ip addresses cannot be used over the internet

unique local ipv6 addresses are defined in

rfc4193.

Hope to help

Giuseppe

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Giuseppe Larosa Wed, 09/17/2008 - 02:57

Hello Garry,

>> Do each device on the LAN have 2 addresses?

yes, two or more if needed; IPv6 is built with this concept multiple IPv6 addresses can be associated to the same interface.

These IPv6 addresses can be of different types.

An IPv6 host for example has to decide what IPv6 address to use when answering an ICMPv6 message for example.

Another example is that IPv6 introduces the concept of on-link and off-link: all prefixes on-link can be reached without going through a router and this is different from IPv4 where you need to go through a router even if the two subnets share the same broadcast medium.

So it can be said that some routing capability is associated with a basic IPv6 host.

Link local is used for example by routing protocols to build adjacencies it is so for OSPFv3 for example.

The support of multiple IPv6 addresses help in deploying a public address change: the router advertisement contain the prefixes on link and their associated lifetime so that hosts can learn new prefixes and be ready to move to another prefix smoothly.

A global ipv6 address is used as source of packets that have to go the internet for example.

Hope to help

Giuseppe

grichardson661 Wed, 09/17/2008 - 03:21

Hi Giuseppe,

Thanks for you reply.

I have a few more questions if that's ok.

The use of IPv6 Link Local address, are these also used for communication between devices sat in the same organisation? and the Global address assigned to a host is used as the hosts source address when communication with hosts on the public internet?

Thanks,

Garry

Giuseppe Larosa Wed, 09/17/2008 - 03:42

Hello Garry,

thanks for your kind remarks

link local are intended for local single link usage only they are not routed/forwarded to another interface by routers.

Some years ago site local addresses were used for organization wide scope but now they have been deprecated.

see the following link

http://www.cisco.com/en/US/technologies/tk648/tk872/technologies_white_paper0900aecd8026003d.pdf

now unique local should be used for organization wide scope

b)yes, a global adddress is used as source for communication wih internet

Hope to help

Giuseppe

grichardson661 Wed, 09/17/2008 - 06:10

Giuseppe, last question.

So whats the difference between Link-Local and Unique Local? You say Unique Local is used for the wide scope, i guess routers make routing decisions on Uniqne local address right and Link Locol dont leave the broadcat domain as in, pass through routers?

That document is very helpful, its just the difference between Link-Local and Unique Local i need to clear up now.

Thanks,

Garry

Correct Answer
Giuseppe Larosa Wed, 09/17/2008 - 11:26

Hello Garry,

your understanding is correct:

link local : confined on a link

unique local: can be routed as used as IPv4 RFC1918 private ip addresses cannot be used over the internet

unique local ipv6 addresses are defined in

rfc4193.

Hope to help

Giuseppe

grichardson661 Wed, 09/17/2008 - 11:51

Hi Giuseppe,

Thanks! That should do it for now. :-)

I feel a little more comfortable now with IPv6. Going to carry on reading documents and tutorials though.

Garry

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