6to4 is known to be unreliable, and Teredo is even worse. When you communicate between 6to4 and Teredo you get all the problems of each combined plus a few more due to complex interactions between the protocols.

Thus it may come as a surprise to you that the answer is: Yes, you can get reliable communication between 6to4 and Teredo.

Both protocols suffer from the same main problem. They rely on third party relays which are underprovisioned and due to their third party nature come with no SLA.

Teredo uses one relay for both directions. 6to4 usually uses two for different directions, but due to the triangular routing in Teredo you end up depending on three 6to4 relays rather than just two. That is a total of four third party relays which you will be depending on - all of which must have enough capacity for your traffic.

But you don't have to rely on third party relays. You can set up your own relay.

Setting up your own Teredo relay

The Teredo relay is the simplest to set up, and it happens to be the most important to your scenario. A Teredo relay needs a single UDP port on a public IPv4 address. Thus you should not deploy the relay on the LAN behind your D-Link router. You should avoid having any 6to4 relay/gateway on the path between your LAN and the Teredo relay. Thus you should not deploy the relay outside the D-Link router.

In short you need a Teredo relay on the D-Link router to make connectivity work reliably. If the D-Link cannot run a Teredo relay, your best option is to replace the D-Link router with a router which can run a Teredo relay. In my experience it will work reliably if you use a Linux machine with Miredo configured in relay mode for the router.

Deploying your own Teredo relay on the D-Link router would not only mean that you no longer rely on a third party Teredo relay. It will also give you a native path between your Teredo relay and your LAN, thus you avoid two of the three third party 6to4 relays as well.

What's left

You would still be relying on a single third party 6to4 relay. A Teredo client need to choose which Teredo server it will be using. The two Teredo clients I know of each have a default configured which will be used if you do not change the configuration yourself. The network path from the Teredo server to your D-Link router will have to pass through a 6to4 relay.

So what you need to do is to choose a Teredo server with access to a reliable 6to4 relay. Ideally a 6to4 relay should be configured on the machine running the Teredo server.

Is this a recommendable configuration?

Installing a Teredo relay on your router is definitely a reliability improvement as long as your router has a public IPv4 address. It will give a significant reliability improvement for any communication with Teredo users, and it will not have any impact on other communication. This is true regardless of whether your router is doing 6to4 or native IPv6.

Using 6to4 on your LAN is however not recommendable as many networks have not installed any 6to4 relays. Hosts on your LAN would often face problems communicating with hosts with native IPv6.

Using a Teredo client is not recommendable either due to all the same reasons that 6to4 isn't. However there are a few cases where Teredo can be useful. Most importantly a Teredo client can connect to hosts on the LAN behind your router (assuming your router has a Teredo relay). And sometimes I have come across CGN deployments working so poorly that Teredo through the CGN is more reliable than TCP through the CGN.

The first thing that should be mentioned about IPv6 subnetting is that a different mode of thought is called for. In IPv4 you usually think about how many addresses you have available and how you can allocate enough of them to each end user. In IPv6 you usually think about how many /64-subnets you have available and how you can allocate them to end users. You almost never worry about how many IP addresses will be used in a given subnet. Except for some special cases like point to point links, each subnet just simply has far more addresses available than it will ever require, so instead you worry only about allocating subnets, not hosts inside them.

IPv6 subnets are usually /64 because that is required in order for SLAAC (stateless address auto-configuration) to work. Even where SLAAC is not in use, there may be other reasons to use /64. For example, there might be some end user devices out there that just assume /64, or else routing subnets narrower than /64 might be inefficient on some routers because the router implementer has optimized the case of /64 or wider routes in order to save routing table memory.

Why is it recommended to use /127 for point to point links?

For the specific case of point-to-point links, /127 is recommended instead of /64 in order to avoid a vulnerability where packets addressed to any one of the quadrillions of unused addresses on the subnet cause unwanted neighbour solicitation requests and table entries that could drown a router. Such misaddressed packets may be malicious or accidental. But even if you actually configure a point-to-point link as /127, some people advocate assigning a whole /64 anyway just to be consistent.

Why would virtual machines be provisioned with subnets narrower than /64?

I don't know specifically why virtual machines would be provisioned with subnets narrower than /64. Perhaps because a hosting provider assumed that a server was like an end-user and required only a single /64 subnet, not anticipating that the server would actually be a collection of VMs requiring an internal routing topology? It could be done also simply as a matter of making the addressing plan easier to memorize: the host gets PREFIX::/64, then each VM gets PREFIX:0:NNNN::/96 where NNNN is unique to the VM and the VM can allocate PREFIX:0:NNNN:XXXX:YYYY as it pleases.

Can I map directly from IPv4 subnets to IPv6 subnets? For instance, does an IPv4 /24 correspond directly to an IPv6 /56 or /120?

From a low-level perspective of how addressing and routing works, the prefix length has the same meaning in IPv6 and IPv4. On that level, you can make an analogy such as "an IPv4 /16 uses half the bits for the network address and half the bits for the host address, that's like a /64 in IPv6". But this comparison is not really apt. Strong conventions have emerged in IPv6 which make the divisions of network sizes look somewhat more like the old world of classful networks in IPv4. To be sure, IPv6 didn't reintroduce classful addressing in which the most significant few bits of the address force a particular netmask, but what IPv6 does have is certain [de facto/conventional] standard network sizes:

• /64: the basic size of a single subnet: LAN, WAN, block of addresses for web virtual hosts, etc... "Normal" subnets are never expected to be any narrower (longer prefix) than /64. No subnets are ever expected to be wider (shorter prefix) than /64 since a /64's worth of host addresses is much more than we can imagine needing.
• /56: a block of 256 basic subnets. Even though current policies permit ISPs to hand out blocks as large as /48 to every end user and still consider their address utilisation well justified, some ISPs may (and already do) choose to allocate a /56 to consumer-grade customers as a compromise between allocation lots of subnets for them and address economy.
• /48: a block of 65536 basic subnets and the recommended size of block that every ISP customer end site should receive.
• /32: the default size of block that most ISPs will receive each time they request more addresses from a regional address registry.

Inside service provider and enterprise networks, many more prefix lengths than these 4 can be seen. When looking at the routing tables of routers inside these networks, IPv4 and IPv6 have much in common including most of the way routing works: routes for longer prefixes override covering routes for shorter prefixes, so it is possible to aggregate (make shorter) and drill down (make longer) routes. Like in IPv4, routes can be aggregated or summarized to larger blocks with shorter prefixes in order to minimize the size of routing tables.

A different question of mapping between IPv4 and IPv6 would be how to harmonize IPv4 and IPv6 assignments on dual-stack machines so that addressing plans can be readily understood. Far that, there are certainly conventions in common use to do this: embed the IPv4 "subnet number" into a portion of the IPv6 prefix, either with BCD (e.g. 10.0.234.0/24 becomes 2001:db8:abcd:234::/64) or binary (10.0.234.0/24 becomes 2001:db8:abcd:ea::/64).

My interfaces have several IPv6 addresses. Must the subnet be the same for all of them?

Absolutely not! IPv6 hosts are expected to be able to be multihomed by having several IP addresses simultaneously that come from different subnets, just like IPv4. If they are autoconfigured with SLAAC then the different subnets might have come from router advertisements from different routers.

Why do I sometimes see a % rather than a / in an IPv6 address and what does it mean?

You would not see one instead of the other. They have different meanings. A slash denotes a prefix (subnet), meaning a block of addresses that all start with the same n bits. An address without a slash is a host address. You may think of such an address as having an implied /128 at the end, meaning all 128 bits are specified.

The percent sign accompanies a link-local address. In IPv6, every interface has a link-local address in addition to any other IP addresses it might have. But the thing is, link-local addresses are always, without exception, in the fe80::/10 block. But if we attempt to talk to a peer using a link local address and the local host has multiple interfaces, how are we to know which interface to use to talk to this peer? Normally the routing table tells us which interface to use for a particular prefix, but here it will tell us than fe80::/10 is reachable via every interface.

The answer is that we must tell it which interface to use using the syntax address%interface. For example, fe80::1234:5678:8765:4321%eth0.

Am I wasting too many subnets? Aren't we just going to run out again?

Nobody knows. Who can tell the future?

But consider this. In IPv6 the number of available subnets is the square of the number of available individual addresses in IPv4. That's really quite a lot. No, I mean really quite a lot!

But still: we are automatically handing out a /32 to any ISP who requests one, we are handing out a /48 to every single ISP customer. Perhaps we're exaggerating and we will squander IPv6 after all. But there is a provision for this: Only one eighth of the IPv6 space has been made available for use so far: 2000::/3. The idea is that if we make a horrible mess of the first eighth and we have to drastically revise the liberal allocation policies, we get to try 7 more times before we're in trouble.

And finally: IPv6 doesn't have to last forever. Perhaps it will have a longer lifetime than IPv4 (an impressive lifetime already and it's not over) but like every technology it will someday stop mattering. We only need to make it until then.