Help understanding some ISP modem options (Ipv6 prefix)

Installing and Using OpenWrt Network and Wireless Configuration
1

(EDIT: warning! I used ffff:: to mask the ip! my bad... it is routable IP, i just chose badly when redacting it)

Hi. I'm trying to map some Ipv6 settings from a very weird modem UX. This is not openWRT specific, only so far as I have to solve this to get my openwrt device online. Goal is to have openWRT correctly advertise things if needed and/or not get in the way of SLAAC working its magic.

The modem is getting public (slightly obfuscated) ip
ffff:14c:7a::547
and I know it is a single /64 (which is plenty for this)

The UI have two options regarding IPv6... one is to show the LAN devices:

Screenshot_20250116_044254
Screenshot_20250116_0442541237×1002 60.3 KB

This translates to "LAN IPv6, Show only active devices"
and the mac address there is the openWRT modem.

Then there's the DHCPv6 screen:

Screenshot_20250116_044339
Screenshot_20250116_0443391167×1084 102 KB

delegated prefix by system: ffff:014c:007a:84fc::/64
Prefix defined by user: [ ]

Server Configuration
The delegated prefix by LAN will be derived from Deletegated Prefix by system, and the initial address will have the same prefix Delegated by LAN

Activate [X]

Initial Address: ffff:014c:007a:84fc:0000:0000:0000:0001

Number of addresses: 4097

TTL: 3600

Activate rapid connect: [ ]
Activate Unicast: [X]
Deactivate Stateless DHCPv6: [X]

Button(Apply changes)

I think i can disable the disabling of stateless, since i want SLAAC... but i'm not sure about the other options. Do I need any of them?

2

To be clear, you have working IPv6 from your ISP, correct?

If so I ask because this is a private IPv6 range, and they must have other NATing for IPv6 to make this work.

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3

Oh I added the ffff: in front as a lame obfuscation. it is public alright.

Should have used something less confusing :sweat_smile:

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4

Yes please :smiley:
I use X or XXXX, for the provider prefix and if I want to obfusicate a local network id, you could use lower x.

But I also stumbled over it, and thought:

what the actual fukk? ffff:? wait what?!

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5

What type of Internet service is this? It looks like LTE where everything is in one /64. You can't do any conventional further routing with a single /64, relay mode is about the best alternative.

If you capture packets on your WAN interface, you will probably see RAs from the modem advertising the same prefix.

6

It's residential cable. Not sure if better or worse...

The fact it gets ip:
ffff:14c:7a::547
and "delegates":
ffff:14c:7a:84fc::/64

Does that mean the whole /64 is being passed down?

7

Yes but you can only "number" one network, like the default lan. Any other additional VLAN (requiring its own prefix), will not get one.

I have no exp. with neighbor-proxy, but I think it can be used to "bundle" multiple downstream interfaces onto a single wan-prefix.

8

Would it be bad if openwrt only get that entire network? I mean, i mostly have a dozen devices in one LAN.

Modem IP:
  ffff:014c:007a:0000:0000:0000:0000:0547
delegate:
  ffff:014c:007a:84fc:0000:0000:0000:0000/64
first client got:
  ffff:014c:007a:84fc:0000:0000:0000:1000

that looks like the modem actually got ffff:14c:7a::/48, but decided to only share the ...:84fc::/64 Does that makes sense?

And more importantly, any option of that UX looks like it would give better choices?

edit: I factory reset the modem and the "prefix delegated by the system" changed:

Modem IP:
  ffff:014c:007a:0000:0000:0000:0000:0547 (same)
delegate:
  ffff:014c:007a:9390:0000:0000:0000:0000/64
first client got:
  ffff:014c:007a::9390:f692:bfff:fe59:13c6

more and more I think i got a /48 and it is just picking one random 64

9

I think its more likely that
ffff:014c:007a:0000: is the prefix for Customer Connected devices, like a modem, and
ffff:014c:007a:9390::/64 is just your single delegated prefix.

If you have only one LAN, then, no that its no problem and the default OpenWrt network config for wan and wan6, would just work out of the box.

PS: Or if you are really curious then set reqprefix 48 or 56 and see what you get.

1 Like
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I put reqprefix=60 and got three /64 ips on wan6 :confused:

one is on the expected range, two others are completely alien.

11

Uh yeah nice! Not. I've heard of such bonkers and broken deployments. Yeah the only solution would be to have just a bunch of half random allocations for each subnet.

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Dumped a settings backup from the modem and i can see it got a /56!

Endereço IPv6 xxxx:14c:7a:84fc:5efa:25ff:fe17:8a3
Prefixo IPv6 xxxx:14c:7a:7a00::/56

edit: never mind, that what someone put as "user defined" range earlier.

13

Yes it looks like every customer's router gets a /128 within the XXXX:XXXX:XXXX:0000/64, then they also get a /64 which can be delegated to one LAN.

Run ifstatus wan6 on your OpenWrt router, or in the GUI look at IPv6 Upstream on the main status page.