0

I recently got assigned a block of IPv6 addresses by my ISP. I've set it up with prefix delegation and it works just fine. When I plug a NIC in on the network it quickly picks up an IPv4 address from DHCP, and gets set up with an IPv6 address in the block, it has a link-local IPv6 address too.

After a few seconds, a second IPv6 address appears. Where is that coming from? How can I find out?

I want to set up firewalls and stuff but it's tricky to be certain about config while addresses are coming seemingly from nowhere!

Immediately

   IPv6 Address. . . . . . . . . . . : 2a02:xxxx:674a:2222:5855:6fce:9c21:fa9b
   Temporary IPv6 Address. . . . . . : 2a02:xxxx:674a:2222:b529:4b04:3529:6828
   Link-local IPv6 Address . . . . . : fe80::5855:6fce:9c21:fa9b%4
   IPv4 Address. . . . . . . . . . . : 10.2.2.40
   Subnet Mask . . . . . . . . . . . : 255.255.254.0
   Default Gateway . . . . . . . . . : fe80::b6fb:e4ff:fe8a:a5ab%4
                                       10.2.2.2

After 5-10 Seconds

   IPv6 Address. . . . . . . . . . . : 2a02:xxxx:674a:2222:5855:6fce:9c21:fa9b
   IPv6 Address. . . . . . . . . . . : fd5e:xxxx:2370:1:5855:6fce:9c21:fa9b
   Temporary IPv6 Address. . . . . . : 2a02:xxxx:674a:2222:b529:4b04:3529:6828
   Temporary IPv6 Address. . . . . . : fd5e:xxxx:2370:1:b529:4b04:3529:6828
   Link-local IPv6 Address . . . . . : fe80::5855:6fce:9c21:fa9b%4
   IPv4 Address. . . . . . . . . . . : 10.2.2.40
   Subnet Mask . . . . . . . . . . . : 255.255.254.0
   Default Gateway . . . . . . . . . : fe80::b6fb:e4ff:fe8a:a5ab%4
                                       10.2.2.2
1

You have something assigning ULA (Unique Local Addressing) addresses on your network. ULA addressing can be used within your network, but it cannot be used on the public Internet.

ULA addresses are in the fc00::/7 range, but the 8th bit is to determine if the addresses are locally assigned. The fc00::/8 range is reserved, and the fd00::/8 range can be locally assigned, with some restrictions. For example, the next 40 bits in the prefix must be randomly assigned.

You can read about ULA in RFC 4193, Unique Local IPv6 Unicast Addresses.


Unfortunately, you have not provided any network information to explain what is assigning those addresses. We would need things like a good network description or diagram, the network device models, and the network device configurations.

2
  • Thanks, I think you're exactly right. I've included a network diagram, any ideas on how I can determine what is giving them out? – Matt F Jun 17 '20 at 15:15
  • Well, you have not identified the network device models, nor provided the network device configurations. This appears to be a residential network, which is off-topic here. Also, if the network devices are consumer-grade devices, those are also off-topic here. You can ask about residential networks and consumer-grade network equipment on Super User. – Ron Maupin Jun 17 '20 at 15:19

Not the answer you're looking for? Browse other questions tagged or ask your own question.