IPv6 Confusion

Owen DeLong owen at delong.com
Tue Feb 17 18:40:50 UTC 2009

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?
There are a lot of different possible answers to that question, many  
of which are accurate.

In general:

It's a 128 bit address.  Routing is done on VLSM, but, generally for  
DNS purposes, these
are expected to be at least on nibble boundaries.

There is an intent to support what is known as EUI-64, which means  
every subnet should
be a /64, however, there are people who number smaller subnets and  
that is supposed
to work, but, it will break certain IPv6 things like stateless  
autoconfiguration (which is

> 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 superseded another.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.
You can subnet it just like IPv4.  Each host does not need it's own  
subnet (/64, not /84 for the most part).
The theory behind /64 subnets was to support a way for a host to use  
what it already knows (MAC
address) and possibly some additional clues (Router Announcement) from  
the wire to configure
its own IPv6 address on an interface.  Whether or not this was a good  
idea is still controversial, but,
whether or not it's how IPv6 is going to work is not.  IPv6 is  
designed to work with Stateless
Autoconfiguration whether we like it or not.  DHCPv6 so far is  
prevented from providing
default router information (or many of the other things you're used to  
having DHCP do)
as it currently stands.

> 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?
It's a fair point.  There is a good FAQ/Wiki on the ARIN web site.   
That may be a good place to

>> 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.
Officially, the best summary I can give is that the subnetting model  
is almost identical to
IPv4, but, all subnets should be at least a /64 (and it's hard to  
imagine a scenario where
a single subnet should be larger, but, it can be supported).

The essential initial guidelines are:

	ISP		/32
			Enough for 4billion ISPs
			Enough for each ISP to support 65,536 /48 customers or 16.7M /56  
customers, etc.
			Larger ISPs can get more than a /32 if needed.

	End Site	/48
			Enough for 65,536 /64 subnets
			Larger organizations can get more than a /48 if needed.

	Single Subnet

			Enough for more hosts that most of us can imagine on a single subnet.
			Support for 64 bit MAC addresses
			Support for stateless autoconfiguration

However, these guidelines can be violated in many circumstances to use  
subnets if you really want to.  I don't recommend it and there's  
really no reason to
do so.

Finally, if we're wrong about all of this, it's OK.  We can renumber  
people into
the other 7/8ths of the IPv6 space that are not yet issued for usage  
with an entirely different numbering scheme.

> 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
Hope the above helps.

> 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.
Unfortunately, other than the guidelines above, most of us are still  
and don't have a lot of op-ex to build on.

> 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."
Your basic bit boundary for a subnet really should be /64.  You  
certainly can put
as many IP addresses on a single host as you wish and there's no  
reason not
to address services as you describe.  There is no longer a concern  
about the
tightness of the subnet since a /64 is the square of the total number  
of hosts
that could be supported on the entire internet without network/broadcast
overhead, etc.

In IPv6, there really is no shortage of addresses and extremely little  
of that ever being a problem, even with the wasteful allocation  
polices we
currently have in place.

Hope that helps,


(Speaking only as and for myself.  This is not an official position or  
from the ARIN AC.  I'm just trying to help.)

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