I have two virtual machines with Ubuntu Server 20.04. One of these virtual machines is a DHCPv6 server, and the other one is a client. I am able to set an IPv6 address on the client, but I cannot ping to and from this address. ip -6 route shows this address but with prefix length of 128. I would like to have the prefix length equals to 64, the same as the server IP address has. When I add to my routing table on the client IP address obtained from the server but with prefix length equals to 64, I can ping between my virtual machines.

This is my /etc/dhcp/dhcpd6.conf file on the server:

default-lease-time 2592000;
preferred-lifetime 604800;
option dhcp-renewal-time 3600;
option dhcp-rebinding-time 7200;
allow leasequery;
option dhcp6.name-servers 3ffe:501:ffff:100:200:ff:fe00:3f3e;
option dhcp6.domain-search "test.example.com","example.com";
option dhcp6.info-refresh-time 21600;

subnet6 fdaa:a:a:a::/64 {
   range6 fdaa:a:a:a::100 fdaa:a:a:a::199;

The last three lines were added by me.

This is my /etc/netplan/00-installer-config.yaml file on the client:

         dhcp4: false
         dhcp6: true
   version: 2

Thank you in advance for any help.

  • 4
    It appears that you are misusing ULA because it does not appear that fdaa:a:a:a::/64 is using a random 40-bit Global ID, and that is a requirement for ULA. "The allocation of Global IDs is pseudo-random [RANDOM]. They MUST NOT be assigned sequentially or with well-known numbers. Locally assigned Global IDs MUST be generated with a pseudo-random algorithm consistent with [RANDOM]. Section 3.2.2 describes a suggested algorithm. It is important that all sites generating Global IDs use a functionally similar algorithm to ensure there is a high probability of uniqueness."
    – Ron Maupin
    Dec 1, 2020 at 14:27
  • What are the logs on the DHCPv6 server showing? Is it Kea, ISC's DHCP-server, something else?
    – Tommiie
    Dec 1, 2020 at 15:26
  • @Tommiie which logs do you mean? I installed dhcp server using command "apt install isc-dhcp-server".
    – user6758
    Dec 1, 2020 at 15:37
  • 4
    Unlike IPv4, IPv6 gets its gateway, prefix, etc. from the RAs.
    – Ron Maupin
    Dec 1, 2020 at 18:20
  • @RonMaupin It seemed to me that using stateless dhcpv6, the gateway and prefix information is obtained from RA, while stateful dhcpv6 allows full configuration. Apparently, I was wrong.
    – user6758
    Dec 1, 2020 at 20:38

1 Answer 1


As per the DHCPd documentation:

In a properly functioning system the client should use a prefix length of 128 and get any on-link information from the RAs from a router.

Since you are using two virtual machines and I assume no IPv6-capable virtual router connects to that virtual network, nobody is sending out those RAs so your client cannot get the correct subnet mask.

Check the link for more information.

  • It seemed to me that stateful dhcpv6 allows full configuration and the RA information is unnecessary. Apparently, I was wrong.
    – user6758
    Dec 1, 2020 at 20:43
  • 2
    @user6758, it is actually the RAs that tell the host whether to use stateful or stateless. RAs are necessary and an integral part of the required NDP.
    – Ron Maupin
    Dec 1, 2020 at 20:55

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