ISC DHCPv6 Server and Stateful Address Allocation

John Jason Brzozowski jjmbcom at
Mon Jan 5 18:43:59 UTC 2009

Question regarding how the ISC DHCPv6 server generates IPv6 addresses it
allocates and the use of range6 where the specified range is longer than the
the prefix specified in subnet6.
Here is an example configuration file:

option dhcp6.preference code 7 = unsigned integer 8;

default-lease-time 120;
ddns-update-style interim;
ddns-domainname "";

subnet6 2001:558:FF10:870::/64 {
range6 2001:558:ff10:870:8000::/65;
option dhcp6.preference 255;

Given the above I would assume that any address allocated from the
range6 2001:558:ff10:870:8000::/65 would have the 65 bit set leaving the
remaining 63 bits to be populated dynamically by the server.  This is now
what I am observing however.  Instead I see the following:

RCV: Advertise message on eth0 from fe80::216:3eff:fe2f:ed6f.
RCV:  X-- Preference 255.
RCV:  X-- IA_NA 3e:18:88:fe
RCV:  | X-- starts 1231168558
RCV:  | X-- t1 - renew  +0
RCV:  | X-- t2 - rebind +0
RCV:  | X-- [Options]
RCV:  | | X-- IAADDR 2001:558:ff10:870:f914:a7c1:42d1:faa1

RCV:  | | | X-- Preferred lifetime 75.
RCV:  | | | X-- Max lifetime 120.
RCV:  X-- Server ID: 00:01:00:01:10:ea:9f:e7:00:16:3e:2f:ed:6f
RCV:  Advertisement immediately selected.

Note the above where all 64 bits are being populated by the DHCP server.  Is
this expected behavior?

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