# IPV6-only questions and thoughts.

## 1clue

Hi,

My Internet connection was in an odd state this morning.  Some sites were up (facebook, mainstream news, netflix, google) but others were not (anything to do with my job, gentoo.org, several other Linux sites).

It turns out my router had an ipv6 address but not an ipv4 address.

I turned everything off for a minute or two, restarted everything in order and tried again to no avail. Called my ISP tech support, who said I had an IPV4 address assigned on their end, but he had no idea why it wasn't showing up on my end. I disconnected my router and plugged in my linux workstation. No joy. I disconnected that and plugged in a mac, again direct ethernet to the cable modem. Again no joy.

I scheduled a service call and hung up, then plugged everything back in the way it was originally configured, and everything works now.

I have some questions and comments:

I thought ipv4 was a subset of ipv6 and that we should be able to get to any ipv4 address from an ipv6-only box?

https://forums.gentoo.org is ipv4-only. Maybe that could be changed?  Will post this on forum feedback.

Pretty much EVERY Linux forum I'm on is ipv4-only.

Why can't my (accidentally) ipv6-only setup reach ipv4 addresses?

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## deagol

IPv6 is quite different from IPv4. They do not depend on each other and can't communicate directly to each other. An IPv6 DHCP server will not assigned you any IPv4 address for sure... You basically tested how much of the Internet is visible for an IPv6 only client and I can imagine that looks still bleak at the moment... According to Google only roughly 16% is IPv6 ready: https://www.google.com/intl/en/ipv6/statistics.html

Now there are ways to allow IPv6 only hosts to address old IPv4 systems, but you need a dual stack system acting as mediator. If your ISP is providing you both, IPv4 and IPv6 addresses it's very unlikely he will also provide an IPv6toIPv4 solution. (In fact getting that working and scaling seems to be hard. Never seen one in action and dual stack down to the customer seems to be the normal approach.)

Once enough systems have IPv6 connectivity IPv4 should be phased out in the Internet and everyone can get rid of the dual stack solutions.

But IPv6 rollout is also needing much more time than expected and it's still not a given you can get IPv6 connectivity at home or in a data center. But anyone not able to do that is now looking quite a bit old fashioned.

The pressure to get IPv6 connectivity is slowly rising and definitely accelerating. But for now IPv4 is still sufficient for most and while I would applaud Gentoo to move to dual stack chances are the IPv6 migration is just picking up steam in most parts of the world.

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## 1clue

I've done some research.

First thing, the ipv4 address space resides in the first 32 bits of the ipv6 address space.  For example, 204.187.15.12 is forums.gentoo.org, and its ipv6 "alternate" ip address notation is ::204.187.15.12.  The traditional ipv6 notation would be ::CCBB:0F0C. You take the 32-bit binary ip address and left-pad with zeroes to 128 bits. So in theory an ipv6-only host can get to any part of the Internet, while ipv4-only hosts must stay in their neighborhood.

Second thing, DHCP for ipv6 is a temporary hack AFAIK. There is such a thing, but it was never intended to be used for very long. It's completely unnecessary for IPV6. It was adapted to IPV6 in order to configure the things that dhcpv4 handles but autoconfiguration in IPV6 doesn't yet have a mechanism for.

I don't have an absolute answer for whether or not a correctly configured and connected IPV6-only client can access an ipv4-only server. My thinking is that you're right, there has to be some dual-stack gateway because the ipv4-only server couldn't understand the address to send things back to.

There's a lot of broad speculation on the net which is posing as fact, and I'm not going to just buy into it without some sort of authoritative proof.

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## deagol

My ipv6 skill/knowledge is a bit rusty, but what I remember agrees more or less with your findings. I'm not so sure that dhcp is slated for disposal, since it has some unexpected new roles in ipv6 and seems to be central to prefix delegation. But I'm not actively following the development and just catch up from time to time... so don't quote me on that   :Confused: 

 *1clue wrote:*   

> I don't have an absolute answer for whether or not a correctly configured and connected IPV6-only client can access an ipv4-only server. My thinking is that you're right, there has to be some dual-stack gateway because the ipv4-only server couldn't understand the address to send things back to.
> 
> 

 That one is really simple: Without some kind of gateway translating the packets it can't work. An ipv4 only client never parse an ipv6 header. Just compare the header and it's more than obvious that any parser designed for ipv4 https://en.wikipedia.org/wiki/IPv4 can never make sense of an ipv6 header https://en.wikipedia.org/wiki/IPv6_packet

Even ignoring the address size increase they are fundamentally different. The only thing an Ipv4 stack can decode is the version, to know it should give up immediately and ignore the packet.

If you want to learn more about ipv6 but don't know where to start: I found the ipv6 certification from Hurrican Electric quite enjoyable: https://ipv6.he.net/certification/.

It's like all the services free and still my source for "static" ipv6 addresses. So if you like learning by doing you may want to check it out.

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## 1clue

I kind of worked out what you said about addresses and packet headers last night.  But thanks.

I went through the HE certification out to the point where I had to set up a mail server. I have absolutely no need or desire to set up a mail server.

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