Due to changes to source IPv6 address selection algorithms made after this blog post was written in 2013, it’s impossible to use ULA addresses in dual-stack networks.

IPv6 Static Addresses and Renumbering

The proponents of Network Prefix Translation for IPv6 (NPT66) usually claim it’s required for one of the two reasons: to implement multihoming without BGP (valid) and to avoid renumbering inside network(s) when the ISP assigns you a new IPv6 prefix. Let’s focus on the renumbering claim today.

A lot of IPv6-focused enterprise engineers have long advised medium-to-large networks to get their own IPv6 address space. Greg Ferro compiled a long list of good reasons to do that, and having your own PI space is not overly expensive anyway – 50€ per year in RIPE region.

Network Renumbering

There are two reasons you might have to renumber your IPv6 network (assuming it’s using public IPv6 addresses like it should):

  • You’re using DHCP prefix delegation with your ISP;
  • You’ve been assigned a static IPv6 prefix but decided to change ISPs (or your ISP decided they have nothing better to do than to annoy their customers).

Most hosts are renumbered automatically if you use SLAAC (it might take a while, so if you’re renumbering your static IPv6 prefix take advance precautions like lowering RA prefix lifetimes), configurations of layer-3 forwarding devices (routers, layer-3 firewalls) and the hosts with static IPv6 addresses have to be changed manually.

Assuming you have a small network with a single layer-3 device (that also acts as a firewall), you can usually perform all renumbering automatically based on IPv6 prefix delegated via DHCPv6 (checking whether your edge router supports that might be a good idea if you’re buying new gear).

If you have more than one layer-3 device in the network, you’ll probably have to do the renumbering process manually (one would hope that vendors would eventually implement hierarchical prefix delegation, but I haven’t seen it implemented yet), but even there a bit of planning goes a long way – general prefixes make life easy for Cisco IOS users (Junos has a regex search/replace that you can use to replace patterns, including parts of IPv6 addresses, throughout the configuration).

Ah, so your preferred vendor doesn’t support easy IPv6 renumbering? Yell at them or vote with your wallet and buy something else.

Server Renumbering

Automatic IPv6 host renumbering works only if the hosts use SLAAC. Hosts with static IPv6 addresses have to be renumbered manually, but even there you could use Unique Local Addresses (ULA – RFC4193) to avoid most of the renumbering hassle – assign a global IPv6 prefix and an ULA prefix to every subnet, use static ULA addresses for your inside servers, and use ULA addresses for all internal communications.

Servers that are not reachable from the Internet don’t have to have static public IPv6 addresses – they should have a static ULA address that their clients can use, and a dynamic public IPv6 address they use to access the Internet if needed (example: software patches).

But what about publicly reachable servers? Technology doesn’t provide a good answer here and so we enter the realms of layer-10 (religious layer). Some people claim it makes perfect sense to have your own public servers even in networks that are so small they can’t justify paying for their own IPv6 address space ... and thus invoke the magic of NPT66 to avoid server renumbering (because it’s so hard to renumber the only server you have in your organization after manually changing its public DNS entry whenever you decide to change your ISP).

I would rather spend $4.95 a month and have a server hosted by someone with reliable infrastructure, which might also increase my web site availability, but that’s just me.


Properly-designed IPv6 networks will use public IPv6 addresses in combination with ULA addresses (when needed). Get used to it. If you’re big enough, get your own IPv6 address space; if you’re too small, buy networking gear that supports easy renumbering.

In my personal opinion, the need for NPT66 to avoid server renumbering in single-homed networks without their own PI IPv6 address space is a totally bogus claim. Organizations this small should not have public-facing servers – web hosting is cheap enough these days.

If you’re an IT geek who wants to be able to hug all his servers, that’s perfectly fine with me – just don’t complain if you have to take care of them every now and then. I prefer to have a web site that keeps running even when I’m on vacation.

More Information

If you’ve just started considering the impact of IPv6 on your network, my Enterprise IPv6 – The First Steps webinar will get you started. Building Large IPv6 Service Provider Networks webinar will give you numerous design guidelines that are also applicable to large enterprise networks (IPv6 for Enterprise Networks is also a pretty good book).

A few more IPv6 webinars are planned for this year and you can get access to all of them with the yearly subscription.

2012-04-04 11:30 GMT: Added information from Sebastian's comment — Junos search/replace functionality.

Latest blog posts in Site and Host Multihoming series


  1. Ivan,

    Security-by-obscurity with the IPv4 NAT is a "good-enough" arrangement for small networks, where there is no need for the big bad Internet to make direct contact with all the unpatched office PCs with RDP enabled on them.

    What's in your opinion the best way of achieving the same with IPv6?

    P.S. I do sort-of buy the "the apps will work better when all hosts cat talk to each other directly"; however when it comes to choosing between the convenience of being able to run crappily designed protocols/apps and providing a however basic level of isolation from the raw Internet without additional effort, I choose the second without batting an eyelid.
  2. Juniper has a simple regex search/replace which can replace patterns in the whole config. Doing a renumbering would be as easy as:

    replace pattern "2001:db8:1:" with "2001:db8:2:"
  3. Thank you! Updated the blog post.
  4. You should really use a stateful firewall on the edge router ... and it should be enabled by default on low-end boxes (like consumer CPE devices).
  5. Thats something that flows through my mind for quite some time now. What is the drawback of using ULAs within the whole network and only address the devices that need direct (no proxy in between) access to the internet with global addresses?

    All internal devices could go through a proxy (how it is done today in a lot of companies) to get to the internet.
  6. > it should be enabled by default on low-end boxes (like consumer CPE devices)

    ...that is, if they have it. Just checked one of those - Apple AirPort Extreme, doesn't have one. Has DHCP, has NAT. No firewall. I suspect situation may be not much better for others. Firewall is tricky, compared to NAT. And more resource-intensive.

  7. Baseline firewall is exactly the same thing as PAT. You need stateful inspection and ALG for FTP and SIP in both cases. Too bad the cheapo vendors don't get it.
  8. Hi Ivan,

    i have seen some NAT "magic" done at the loadbalancer or stateful firewalls because of asymmetric routing behind that barrier. otherwise the returning packet would not go through the same firewall and would be dropped.
    How would you solve this problem with IPv6?


  9. i know my question is not really related to the static addresses and renumbering topic but the question came just to my mind...
  10. Did you have to open that particular can of worms :-P Every time I have to think about this, I get frustratingly mad at the shortsightedness IETF had 20 years ago ... not to mention the TCPng fiasco.

    Load balancing in IPv6 works exactly the same as in IPv4. Nothing you can do to change it.

    You might make it more anycast-ish with LISP (and lose some of the fast failure detection and dynamic balancing capabilities), but the problem is that it would work well only if you have enough ITRs.
  11. the 50euros per year per prefix is based on that you do have a sposonring LIR ie your ISP. And you do have to go thru the process of applying for a /48 from RIPE. So the costs /time involved would be a bit more expensive than that.
  12. Absolutely true. Thanks for the feedback!
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