IPv6 ULA for redundancy

2016 November 30
by Daniel Lakeland

The IPv6 standard has a concept called ULA (Unique Local Address) similar in nature to the or address spaces in IPv4. For IPv6 these are in the address space fc00::/7. They are addresses that are defined only locally within an organization and don’t route on the wide internet. In general, it seems like these addresses are a bad idea to use reflexively. But what are some actual use cases?

One that I can think of is to deal with the 6rd addresses typically handed out by consumer ISPs. The way these work is that each ISP has a prefix, and then your router creates a sub-prefix by taking the ISP prefix and appending your dynamic IPv4 address. If the ISP prefix is short enough, you can have a few bits of address space to play with for yourself. A 6rd prefix looks like


Well, as you can probably guess, every time your power goes out for a couple of hours you might lose your IPv4 DHCP lease and now your whole network needs to be renumbered when you come back up with a new ipv6 prefix.

Ideally, when you sign up with your ISP, they’d give you a fixed static IPv6 /56 or even /48 prefix, which you would keep until you decide to move to a different ISP, but instead of that administrative hassle, they’ve invented a way to use the existing IPv4 DHCP infrastructure, which makes their lives easier, but your life less certain.

Sure, responding to new prefixes is do-able. But also, what about ISP outages? Just because your ISP goes down when a squirrel chews through your connection, doesn’t mean you want to lose access to say your printer or scanner or file-share within your home or small business (ok, sure you’ve got an ipv4 set up anyway… but honestly that won’t be forever).

Enter the ULA. You create a random prefix in the ULA space (get one randomly generated here) and then you set this up as an additional prefix for your local network. Now you can provide your laser printer or Samba share or web/security camera with a static local address that doesn’t depend on what your ISP (or the squirrels!) had for breakfast, and your other hosts can auto-configure via SLAAC so that they can access these printers/cameras/shares via the ULA prefix.

If you’re big enough to have your own assigned IPv6 prefix, then great, you can use those numbers, but if your ISP is someone like ATT who is using 6rd and your IPv6 prefix is inevitably going to depend on some IPv4 DHCP lease that is totally unpredictable, then ULA can give you a predictable redundant local network that always looks the same regardless of what happens on the wide internet. That’s a good thing for the majority of us.


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