IPv6 Confusion

Fred Baker fred at cisco.com
Tue Feb 17 18:04:26 UTC 2009

You already have a fair bit of information, but the short answer to  
your question is...

Apart from a few special purposes addresses (see RFC 4291), IPv6  
addresses are a cross between IPv4-style CIDR addressing and XNS/IPX/ 
ISO-style network+host addressing. Bits 0..63 of the address are a  
CIDR prefix; there are various guidelines on what prefix lengths  
should be allocated in various cases, but they are just that -  
unenforceable and mutable. Bits 64..127 are a host part, which may be  
a MAC Address, a random number, or pretty much anything else. The key  
thing is that the host part is unique on a LAN.

There are probably four important prefix lengths in the world -  
prefixes that might have special meaning, prefixes that get allocated  
by IANA, prefixes that get allocated by RIRs, and prefixes that get  
allocated to customers of ISPs. In general, IANA is treating IPv6 much  
like IPv4 - making sure that the RIRs have enough to work with. The  
RIRs are also treating IPv6 much like IPv4, with the exception that  
the rules require a lot less hoop-jumping. If folks need prefixes,  
they hand them out. The one difference is that they are trying to  
avoid PI addressing, as that is one of the major contributors to the  
current explosion of the route table (the others being "long prefixes"  
and "traffic engineering"). What to replace PI addressing with is a  
topic of ongoing discussion - various ideas have been proposed but all  
require some change to some business model or sacred cow somewhere,  
meaning that any idea that is viable gets dismissed pretty rapidly,  
something the folks who buy memory for routers will eventually pay the  
price of unless that nonsense passes. Edge networks are edge networks;  
they tend to assign subnets, or campuses with subnets in them.

On Feb 17, 2009, at 8:59 AM, Carl Rosevear wrote:

> So, I understand the main concepts behind IPv6.  Most of my peers  
> understand.  We all have a detailed understanding of most things  
> IPv4.  I have Googled and read RFCs about IPv6 for HOURS.  That  
> said, to quickly try to minimize people thinking I am an idiot who  
> asks before he reads, I need some answers.  First of all, several of  
> my friends who feel they are rather authoritative on the subject of  
> things network-related have given me conflicting answers.  So what's  
> the question? ...
> How does IPv6 addressing work?
> I know it's been hashed and rehashed but several orgs I am  
> associated with are about to ask for their allocations from ARIN and  
> we are all realizing we don't really know how the network / subnet  
> structure trickles down from the edge to the host.  We really don't  
> have a firm grasp of all of this as there seems to be multiple  
> options regarding how many addresses should be assigned to a host,  
> if the MAC address should be included in the address or if that is  
> just for auto-configuration purposes or what the heck the deal is.   
> There are a lot of clear statements out there and a lot that are  
> clear as mud.  Unfortunately, even when trying to analyze which RFC  
> superseded another.  Can I just subnet it all like IPv4 but with  
> room to grow or is each host really going to need its own /84 or  
> something?  I can't see why hosts would need any more addresses than  
> today but maybe I'm missing something because a lot of addressing  
> models sure allow for a huge number of unique addresses per host.
> My buddy and I are about to go to Barnes and Noble, not having and  
> luck with standard internet media but then we realized...  how will  
> we know if any of that is really what we are looking for either?
>> From what I can tell, this may still be a question of great  
>> debate.  Everyone seems to act like they know exactly what's going  
>> on but behind closed doors admits that they don't really know x, y,  
>> or z.  I realize this is typical of my industry and even myself  
>> from time to time.  J
> But so I am truly reaching out here.  What is the deal with IPv6  
> addressing and subneting? Where is the official guide to this new  
> galaxy?  I will be sure to pass this information on to my equally  
> less clueful peers to the benefit of all of us that are making this  
> transition.
> There are people here at my company that seem to get it but can't  
> seem to explain it clearly to me.  To me, its basically just larger  
> addressing space with some new logical boundaries....  But there are  
> so many discussions of potential addressing methods that I am  
> confused.   I know from my lab setups that I can "make it work" but  
> I'd like to "do it right".  J
> I've been doing this for over 10 years now...   IPv4 is native to  
> me.   If you can point me in the direction of some good,  
> authoritative information or even say "Dood, go get IPv6 for  
> dummies", that's fine I just need to know where to find some good  
> information.
> Can someone say "well, you know how it would be nice to have like  
> 100 different addresses on hosts to differentiate services and blah  
> blah....  Well now that's what you account for and so then you know  
> how a /24 almost always ends up being tight in IPv4?  Right, so  
> think of your basic bit boundaries that you adhere to as /??  
> And /???   In IPv6."   Or "Throw all that old thought out the  
> window.    Now its kind of like how the Ford Probe is actually a  
> Mazda...  ummm....  Yeah I can't really explain it either but it  
> makes sense.  Here read this book and it'll make sense to you too."
> Respectfully yours,
> Carl Rosevear

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