Seeking clarificationon IPv6 Link & Site local addresses please

Answered Question
Dec 29th, 2007

Hi All

I posted a question yesterday on the "Getting started with LAN's" forum re: IPv6 Link Local & Site Local addresses which I don't want to cross post. However, just in case there are any IPv6 gurus here who do not post on the "Getting Started" forum I though I would ask if you could have a look and see if you could provide an answer.

Best Regards & Thanks in Advance,

Michael.

Correct Answer by swmorris about 9 years 1 month ago

I don't look at every forum, so I know I missed that one... I replied though, and just to be helpful for the others waiting in suspense:

Original:

Hi All

Just wondering if anybody with IPv6 knowledge can put me straight. I am currently studying for my CCNP (BSCI) and am just going over the IPv6 stuff again.

Now I understand that for the CCNP I will likely just need to know the basic theory of IPv6, however I cannot get my head around the Link-Local and Site-Local addresses.

Routing TCP/IP Vol 1 (Jeff Doyle) 2nd edition, page 54, has a table showing the high order bits of various IPv6 addresses and lists the following.

Link-Local = 1111 1110 10 which equates to FE80::/10

Site-Local = 1111 1110 11 which equates to FEC0::/10

Now the part I can not get my head around is this.

The first 10 high order bits equate to 2 1/2 (10 bits) nibbles, but for the Link-Local address to be FE80 would require 4 nibbles (16 bits) or

1111 1110 1000 0000. Like wise with the Site-Local address of FEC0 which would be 1111 1110 1100 0000. This would mean that the prefix should be /16 if remaining 6 bits can not be turned on.

So I am trying to understand what happens to the remaining 6 bits in these Local IPv6 addresses or to the IPv6 addresses from FE81 to FEBF and from FEC1 to FECF? Are they just ignored?

If so it would appear that nothing was learned from what happened with the initial wasteful assignment of IPv4 addresses!!

Can anybody put me out of my misery please :)

Best Regards,

Michael

Response:

It's a little trickier than that. I never knew why they listed them as /10's because you're right, it implies things can be set however you want, but they can't.

Link local is specifically:

FE80:0000:0000:0000:: (eui-64 or defined address)

Site local is specifically:

FEC0:0000:0000:(subnet):: (eui-64 or defined address)

To further muck things up, the site local addressing has been deprecated in the real world. There is a new set called ULA (unique local address) out there, but it won't be covered in the CCIP exams yet, so don't stress out on that part! (grin)

HTH,

Scott

smorris@ipexpert.com

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Correct Answer
swmorris Sat, 12/29/2007 - 07:42

I don't look at every forum, so I know I missed that one... I replied though, and just to be helpful for the others waiting in suspense:

Original:

Hi All

Just wondering if anybody with IPv6 knowledge can put me straight. I am currently studying for my CCNP (BSCI) and am just going over the IPv6 stuff again.

Now I understand that for the CCNP I will likely just need to know the basic theory of IPv6, however I cannot get my head around the Link-Local and Site-Local addresses.

Routing TCP/IP Vol 1 (Jeff Doyle) 2nd edition, page 54, has a table showing the high order bits of various IPv6 addresses and lists the following.

Link-Local = 1111 1110 10 which equates to FE80::/10

Site-Local = 1111 1110 11 which equates to FEC0::/10

Now the part I can not get my head around is this.

The first 10 high order bits equate to 2 1/2 (10 bits) nibbles, but for the Link-Local address to be FE80 would require 4 nibbles (16 bits) or

1111 1110 1000 0000. Like wise with the Site-Local address of FEC0 which would be 1111 1110 1100 0000. This would mean that the prefix should be /16 if remaining 6 bits can not be turned on.

So I am trying to understand what happens to the remaining 6 bits in these Local IPv6 addresses or to the IPv6 addresses from FE81 to FEBF and from FEC1 to FECF? Are they just ignored?

If so it would appear that nothing was learned from what happened with the initial wasteful assignment of IPv4 addresses!!

Can anybody put me out of my misery please :)

Best Regards,

Michael

Response:

It's a little trickier than that. I never knew why they listed them as /10's because you're right, it implies things can be set however you want, but they can't.

Link local is specifically:

FE80:0000:0000:0000:: (eui-64 or defined address)

Site local is specifically:

FEC0:0000:0000:(subnet):: (eui-64 or defined address)

To further muck things up, the site local addressing has been deprecated in the real world. There is a new set called ULA (unique local address) out there, but it won't be covered in the CCIP exams yet, so don't stress out on that part! (grin)

HTH,

Scott

smorris@ipexpert.com

keeleym@o2.ie Sat, 12/29/2007 - 08:55

Hi Scott

Cheers for the response and the information. It's much appreciated.

I think it is really incredible that the powers that be who developed IPv6 and decide how it will be used appear to have learned nothing from the problems which occurred with IPv4 and are willing to apparently waste vast chunks of IPv6 address space.

Looks like history repeating itself.

Best Regards & Best Wishes for 2008 and beyond,

Michael

swmorris Sat, 12/29/2007 - 10:22

hehehehe... Yeah, there's a bit of that, but on the other hand while 4.2 billion addresses seemed like a lot, 340 dodecillion is a bunch more! :)

Statistical avoidance! Same issues, but with a much larger pool make it unlikely to have any problems in the foreseeable future. On the other hand, by the time there are problems (United Federation of Planets, etc.) it'll be someone else's problem to fix! (smirk)

Scott

smorris@ipexpert.com

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