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> The problem is how do you know how many subnets the ISP is willing to give you? Unfortunately there’s no way of finding this This was by far the most frustra

> The problem is how do you know how many subnets the ISP is willing to give you? Unfortunately there’s no way of finding this This was by far the most frustrating part of configuring IPv6 on my home network. Every IPv6 guide out there assumes you already know the PD size for your ISP, and I couldn't find any tool that would let me test different sizes. Even if I had known > you can run odhcp6c manually with the -P option if you have to probe your ISP to find out what size of prefix you can get , I don't have an OpenWRT router so that wouldn't have helped me. In the end I just had to guess and hope that if it didn't work it was in fact the wrong PD size and not some other misconfiguration on my part.

Smar 6y ago
If I’m not wrong, can’t you just listen router advertisements to get correct subnet? I just configured similar setup using RA and DHCPv6 for my personal network.
- thedanbob 6y ago
  I don't know, and I wouldn't know how to do that either. In the end I found out that if I monitored the dhcp6c.log file on my router and tried different PD sizes I could see whether it was working or not.
- Dagger2 6y ago
  You can listen to RAs on your router's WAN interface, which may tell you the WAN subnet, but they won't tell you anything about the routed prefix which is what you use for networks on the LAN side of the router. The only way to get any info about that is to do DHCPv6-PD requests (or ask the ISP).
  - takeda 6y ago
    Hmm, I thought these prefixes a subset of the CIDR provided through RA on WAN? I am kind of in similar situation as parent poster. My ISP supposedly provides IPv6, but from the RA announcements it looks like it is /64. My assuption was that I supposed to split it myself into smaller pieces. It's also sucks since they are providing /64 which is the smallest CIDR that SLAAC requires. When I called my ISP they said that I only have IPv4, so it's not like I can get much help with IPv6. Although I know IPv6 works, because when I connected a box and told it to configure itself through RA it did and I was able to ping ipv6 hosts.
    - Dagger2 6y ago
      Nope, the WAN and LAN networks need to use different, non-overlapping network ranges. The only automated way to get the LAN prefix is to ask the ISP via DHCPv6-PD. Routing in v6 is exactly the same as in v4. If your ISP gives you 203.0.113.42/24 on the WAN, you can't turn around and use other parts of 203.0.113.x on the LAN. The only difference is that in v6 you actually get a routed prefix, and that DHCPv6-PD exists to automate it.

Smar 6y ago

If I’m not wrong, can’t you just listen router advertisements to get correct subnet? I just configured similar setup using RA and DHCPv6 for my personal network.

thedanbob 6y ago
I don't know, and I wouldn't know how to do that either. In the end I found out that if I monitored the dhcp6c.log file on my router and tried different PD sizes I could see whether it was working or not.
Dagger2 6y ago
You can listen to RAs on your router's WAN interface, which may tell you the WAN subnet, but they won't tell you anything about the routed prefix which is what you use for networks on the LAN side of the router. The only way to get any info about that is to do DHCPv6-PD requests (or ask the ISP).
- takeda 6y ago
  Hmm, I thought these prefixes a subset of the CIDR provided through RA on WAN? I am kind of in similar situation as parent poster. My ISP supposedly provides IPv6, but from the RA announcements it looks like it is /64. My assuption was that I supposed to split it myself into smaller pieces. It's also sucks since they are providing /64 which is the smallest CIDR that SLAAC requires. When I called my ISP they said that I only have IPv4, so it's not like I can get much help with IPv6. Although I know IPv6 works, because when I connected a box and told it to configure itself through RA it did and I was able to ping ipv6 hosts.
  - Dagger2 6y ago
    Nope, the WAN and LAN networks need to use different, non-overlapping network ranges. The only automated way to get the LAN prefix is to ask the ISP via DHCPv6-PD. Routing in v6 is exactly the same as in v4. If your ISP gives you 203.0.113.42/24 on the WAN, you can't turn around and use other parts of 203.0.113.x on the LAN. The only difference is that in v6 you actually get a routed prefix, and that DHCPv6-PD exists to automate it.