Had an interesting issue in recent weeks, which affected IPv6 connectivity on my BBSs. For almost 10 years, the connection has been trouble free, except for some issues when moving house in 2013. In 2017, upgrading to the NBN (runs VDSL here) led to a vast improvement in service. Most importantly, since 2013, when we moved to this address, my LAN IPv6 prefix hadn't change. The LAN used the first /64 from my /56, with the WAN interface getting an address from another /54.

Sometime in the near past, ,y LAN prefix changed to the second /64 in my range, which baffled ne. Rebooting the router sometimes got me back to the first /64, but at the time I didn't know what was going on. All I knew is everything worked again. When IPv6 failed within 12 hours of a reboot, I took a closer look and discovered that my prefix had changed to the second /64 in my range. So I reconfigured the BBSs and other hosts that require static addresses, as well as their corresponding DNS entries, and all was well. Next morning, the LAN prefix had reverted to the first /64!

At this point, I considered phoning the ISP, but it was getting a bit late, so I tinkered. Went into the router and found there were 3 (relevant to me) options for obtaining an IPv6 address. The first (and the option I was originally set to) looks for router adverisements first, then tries DHCPv6, and finally assigns an address from the first subnet if all else fails. I figured there's something going on between router advertisements and DHCPv6.

The second option tries only DHCPv6. I didn't try this one.

The third option uses only router advertisements, or assigning an address from the first /64 if that fails. I switched to this option.

Since making the switch, my LAN prefix seems stable (touch wood) on the second /64 of my range. Time will tell, but hopefully I've stabilised things on the new address range. Looks like my router started using DHCPv6 only intermittently, and DHCPv6 assigns a different WAN address. I'm only guessing, as I can't see what's happening on the ISP side. Maybe they're tweaking things too.

99.9% of customers wouldn't notice this issue (and it never affected web browsing and other client only activities on machines using SLAAC). I only noticed and have issues, because I have a lot of things listening on fixed IPs.


... Hangnail: Coat hook.
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* Origin: Freeway BBS Bendigo,Australia freeway.apana.org.au (3:633/410)