A /64 prefix and SLAAC can only really be applied to a single link in
your network, so if you wanted to separate your network into multiple
links (ie: not bridging) then you would use a shorter prefix to get the
routing right between each of those links.
For example, the IPv6 prefix generated by your router might be
fd83:af19:9ef::/60, but your your ethernet devices would see
fd83:af19:9ef:1::/64 for SLAAC, and your WiFi devices might see
fd83:af19:9ef:2::/64 for SLAAC. Because they are both subnets of the
broader /60 prefix, your router can advertise itself as the router for
all of the links in your home network.
Cheers,
Owen
P.S. This is all hypothetical, I haven't actually played with this
option to see what it does... but this is the typical use case for IPv6
prefixes shorter than 64-bits and ULAs.
On 14-05-02 11:14 AM, Gert Doering wrote:
> Hiya,
>
> I've installed "trunk (r40576)" on a few boxes because I want to play
> around with homenet (hnetd / package hnet-full).
>
> Before I even get there, I'm wondering about something. The sample
> "/etc/config/network" file has an option in there which confuses me:
>
> config interface 'lan'
> option ifname 'eth1'
> # option type 'bridge'
> option proto 'static'
> option ipaddr '192.168.1.1'
> option netmask '255.255.255.0'
> option ip6assign '60'
>
> what is "option ip6assign" good for, and why does it default to "60"?
> ("option bridge" commented out by me, as hnetd supposedly does not
> like bridges)
>
> The effect it has is that the interface in question receives a /60 as
> IPv6 network connected to it:
>
> root@OpenWrt:/etc/config# ifconfig -a
>
> eth1 Link encap:Ethernet HWaddr 10:FE:ED:E6:5F:32
> inet addr:192.168.10.1 Bcast:192.168.10.255 Mask:255.255.255.0
> inet6 addr: fe80::12fe:edff:fee6:5f32/64 Scope:Link
> inet6 addr: fd83:af19:9ef::1/60 Scope:Global
> inet6 addr: 2001:608:0:c10::1/60 Scope:Global
>
> ... which is not exactly "what the IETF says should be on a LAN" - but
> some other parts of the system see the prefix as /64, like when sending
> out a RA on that LAN
>
> 17:51:19.741002 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 192)
> fe80::12fe:edff:fee6:5f32 > ff02::1: [icmp6 sum ok] ICMP6, router
> advertisement, length 192
> hop limit 0, Flags [managed, other stateful], pref medium, router
> lifetime 1800s, reachable time 0s, retrans time 0s
> source link-address option (1), length 8 (1): 10:fe:ed:e6:5f:32
> mtu option (5), length 8 (1): 1500
> prefix info option (3), length 32 (4): 2001:608:0:c10::/64, Flags
> [onlink, auto], valid time 2817s, pref. time 1017s
> prefix info option (3), length 32 (4): fd83:af19:9ef::/64, Flags
> [onlink, auto], valid time 7200s, pref. time 1800s
>
> ... which is perfectly correct, as SLAAC only works for /64.
>
>
> So, well, my question boils down to "why is that default there?", and
> "what effects does this option have, besides assigning /60 prefixes to
> LAN interfaces?".
>
> (As a side note: I really like the way IPv6 has gotten integrated into
> newer releases. Plug in that thing, received DHCPv6-PD from upstream
> routers, offer v6 to connected LANs, off you go...)
>
> gert
>
>
>
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