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Stateful DHCPv6 - Why am I getting a /128 Subnet?

DHCP Server
R1(config)#ipv6 dhcp pool [DHCP]2001:1::/64
R1(config-dhcpv6)#address prefix 2001:1::/64
R1(config-dhcpv6)#dns-server 2001:1::1
R1(config)#int fa0/0
R1(config-if)#ipv6 address 2001:1::1/64
R1(config-if)#ipv6 dhcp server [DHCP]2001:1::/64

DHCP Client
R2(config)#interface FastEthernet0/0
R2(config-if)#ipv6 enable
R2(config-if)#ipv6 address dhcp

R2#show ipv6 interface fa0/0
FastEthernet0/0 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::C802:DFF:FED4:0
  No Virtual link-local address(es):
  Global unicast address(es):
    2001:1::4D7E:3B4A:2A48:E093, subnet is 2001:1::4D7E:3B4A:2A48:E093/128
  Joined group address(es):
  MTU is 1500 bytes
  ICMP error messages limited to one every 100 milliseconds
  ICMP redirects are enabled
  ICMP unreachables are sent
  ND DAD is enabled, number of DAD attempts: 1
  ND reachable time is 30000 milliseconds (using 30000)
  ND advertised reachable time is 0 (unspecified)
  ND advertised retransmit interval is 0 (unspecified)
  ND router advertisements are sent every 200 seconds
  ND router advertisements live for 1800 seconds
  ND advertised default router preference is Medium
  Hosts use stateless autoconfig for addresses.


Why am I getting a /128 Subnet?

Isn't it supposed to be:

  Global unicast address(es):
    2001:1::4D7E:3B4A:2A48:E093, subnet is 2001:1::/64


What did I do wrong, and how do I fix it?

13 Replies 13



Well... Prefix Delegation is actually talking about a different thing.


The DHCPCLIENT is actually getting its s0/0 IPv6 EUI-64 Address using SLAAC.

The DHCPSERVER do advertise the Prefix, but just the Prefix alone.


DHCPCLIENT receives it on s0/0, and saves it in "ipv6 dhcp client pd [PREFIX]X".

DHCPCLIENT then uses [PREFIX]X on fa0/0, and manually configure the host bits.


Once DHCPCLIENT's fa0/0 have a complete address, CLIENT1 uses SLAAC again to get the IPv6 EUI-64 Address


So... Thanks, but no thanks.





DHCPv6 does not provide prefix like DHCPv4 provide subnet mask.


The client gets prefix information from router advertisment (RA). RA is generated (and Cisco router is joining FF02:2 multicast group) when "ipv6 unicast routing" command entered.

I have also same question about prefix size with the DHCPv6. 

I could find the reason in here. 

Unfortunately, i don't have IPv6 router. So, i can't receive RA packet from router and i can't check prefix updating.

I installed  DHCPv6 server in the Ubuntu. And add prefix option in the configuration.

The prefix is   prefix6 2001:db8:bad:a55:: 2001:db8:bad:a55:: /64. 

      default-lease-time 120;
      max-lease-time 180;
      log-facility local7;
      subnet6 2001:db8:bad:a55::/64 {
      option 2001:4860:4860::8888, 2001:4860:4860::8844;
      option dhcp6.domain-search "";
      option dhcp6.sip-servers-addresses fe80::b261:xxxx:xxxx:21ef;
      range6 2001:db8:bad:a55::100 2001:db8:bad:a55::199;
      range6 2001:db8:bad:a55::/64 temporary;
      prefix6 2001:db8:bad:a55:: 2001:db8:bad:a55:: /64;


After that, i captured advertise packet and found the Prefix delegation information in the packet

          Message type: Advertise (2)
          Transaction ID: 0xf48873
          Identity Association for Prefix Delegation
          Option: Identity Association for Prefix Delegation (25)
          Length: 41
          Value: 000000010000000000000000001a00190000004b00000078...
          IAID: 00000001
          T1: 0
          T2: 0
          IA Prefix
          Option: IA Prefix (26)
          Length: 25
          Value: 0000004b000000784020010db80bad0a5500000000000000...
          Preferred lifetime: 75
          Valid lifetime: 120
          Prefix length: 64
          Prefix address: 2001:db8:bad:a55::


It says Prefix address 2001:db8:bad:a55::/64. 

But, DHCPclient always have prefix /128 without update.

Can DHCPclient  prefix size be updated with IA_PD value  by prefix delegating?





Deepak Kumar


Did you configure "ipv6 unciast-routing" command?



Deepak Kumar



Deepak Kumar,
Don't forget to vote and accept the solution if this comment will help you!

Is  the command only possible in the Cisco Router?



This is for Cisco router. Are you looking for any other device?



Deepak Kumar

Deepak Kumar,
Don't forget to vote and accept the solution if this comment will help you!


I'm not network administrator. So, i can't access Cisco router.

But, my question seems doesn't relate with the routing operation.

It is theoretical question about DHCPv6.

This is the IP addresses lists of my test machine. 

    sh-4.3# ip addr show eth0
   2: eth0: <BROADCAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
   link/ether xx:xx:xx:06:21:ef brd ff:ff:ff:ff:ff:ff
   inet 1xx.1xx.140.65/24 brd 1xx.1xx.1xx.255 scope global eth0
   inet6 2001:1:1:1104::140/128 scope global dynamic
   valid_lft 85sec preferred_lft 40sec
   inet6 2001:1:1:1104:xxxx:xxxx:fe06:21ef/64 scope global dynamic
   valid_lft 2591962sec preferred_lft 604762sec
   inet6 fe80::xxxx:xxxx:fe06:21ef/64 scope link
   valid_lft forever preferred_lft forever


The 2001:1:1:1104::140/128 is assigned by the DHCPv6 server

And the 2001:1:1:1104:xxxx:xxxx:fe06:21ef/64 is the global address which is generated after Router advertisement message.

The prefix from the route is 2001:1:1:1104 /64.

But, the prefix of DHCPv6 address keeps size 128. It's not changed. 

I understood that it would be changed when the client received prefix value and size from the Router.





The prefix length is 128 when the address is provisionned via DHCPv6 and it will remain 128 whether you receive a router advertisement or not. The host will install an entry for the prefix (/64) if the router advertises it with the on-link bit set. Otherwise, all traffic destined to other hosts on the same subnet would need to traverse the router.



Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
México móvil: +52 1 55 8312 4915
Cisco México
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Cuauhtémoc, Juárez
Ciudad de México, 06600

Thank you for your teaching. I understood on-link and autonomous flag in the RA message. 

If the on-link sets with 1, the host sends packet directly when the destination is matched with prefix.

If it sets with 0, the host sends packet to the router when the destination is matched with prefix.

It means that the router can send numerous prefix information to the host.


If the autonomous flag is set with 1, the host creates a new address based on this prefix.

If it sets with 0, don't create a new address based on this prefix.





I found new article which gives detailed explanation about IA_PD.

I could correct my misunderstood about IA_PD function. 

I hope this link helps people who have similar question with me.





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