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i'm learning IPv6 so i was looking at the output of the ipconfig /all command on my pc. I have already disabled the IPv6 temporary address feature in order to avoid the many lines about these addresses.

My output looks like this (real values replaced with xxxx):

Scheda Ethernet Ethernet:
Suffisso DNS specifico per connessione: homenet.telecomitalia.it
Descrizione . . . . . . . . . . . . . : Realtek PCIe GBE Family Controller
Indirizzo fisico. . . . . . . . . . . : xx-xx-xx-xx-xx-xx
DHCP abilitato. . . . . . . . . . . . : Sì
Configurazione automatica abilitata   : Sì
Indirizzo IPv6 . . . . . . . . . . . . . . . . . : fd0c:2759:291d:0:xxxx:xxxx:xxxx:xxxx(Preferenziale)
Indirizzo IPv6 . . . . . . . . . . . . . . . . . : fd5a:dde5:fb8c::xxxx(Preferenziale)
Lease ottenuto. . . . . . . . . . . . : lunedì 23 ottobre 2017 15:07:32
Scadenza lease . . . . . . . . . . .  : giovedì 29 novembre 2153 22:18:02
Indirizzo IPv6 . . . . . . . . . . . . . . . . . : fd5a:dde5:fb8c:0:xxxx:xxxx:xxxx:xxxx(Preferenziale)
Indirizzo IPv6 locale rispetto al collegamento . : fe80::6c1c:7033:c528:fd5a%10(Preferenziale)
Indirizzo IPv4. . . . . . . . . . . . : 192.168.1.68(Preferenziale)
Subnet mask . . . . . . . . . . . . . : 255.255.255.0
Lease ottenuto. . . . . . . . . . . . : lunedì 23 ottobre 2017 15:07:31
Scadenza lease . . . . . . . . . . .  : lunedì 23 ottobre 2017 21:07:30
Gateway predefinito . . . . . . . . . : 192.168.1.1
Server DHCP . . . . . . . . . . . . . : 192.168.1.1
IAID DHCPv6 . . . . . . . . . . . : 55633225
DUID Client DHCPv6. . . . . . . . : 00-01-00-01-1F-88-7C-3A-50-E5-49-CA-EE-F7
Server DNS . . . . . . . . . . . . .  : fe80::c66e:1fff:fea2:c01c%10
                                       8.8.8.8
                                       8.8.4.4
NetBIOS su TCP/IP . . . . . . . . . . : Attivato

I do not understand why i have multiple private IPv6 addresses. I mean clearly the one starting with fe80 is a link local address so it's ok but i have 3 other addresses that look like private. The 1st one beginning with fd0c and the 3rd starting with fd5a have the same interface ID, the 2nd begins with fd5a but the interface ID is totally different.

Why should i have multiple private IPv6 addresses on a single interface? Is this something normal? How are they generated if the Interface ID is not the same in all 3 addresses?

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    The question is why you have ULAs ("private" IPv6 addresses) in the first place - if you want public internet access, you really shouldn't have them, NAT between those and global IPv6 addresses is broken. Are you behind a home router? What is your internet provider?
    – dirkt
    Commented Oct 23, 2017 at 16:45
  • The ISP is Telecom Italia, of course i am behind a home router. The router has a public IPv4 address but not a public IPv6 address so i suppose that my ISP does not provide home users with an IPv6 addressing plan. I think that everything is carried on thanks to the NAT since the only public address i have is the IPv4 public address of the router interface. So basically the ISP's modem is the only one with a public address so it is the central node of my network, it will forward packets outside my local network and simultaneously receive and route packets towards my local devices.
    – matteof93
    Commented Oct 23, 2017 at 17:45
  • My question is not why i do not have a public IPv6 address, i would only like to know why i have multiple ULAs and, maybe, why do i have ULAs at all since i could simply use the link local IPv6 address to communicate with devices on my local link.
    – matteof93
    Commented Oct 23, 2017 at 17:50
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    I understood the question. :-) Google shows that Telecom Italia seems to have developed some IPv4 to IPv6 migration scheme, though hits are from around 2011 so I have no idea if this is still in effect. I'm afraid only Telecom Italia will be able to tell you why they need three IPv6 ULA addresses for this scheme, unless someone else is able to figure out details. And no, you can't simply use the link-local FE80 address, it won't be routed (by design). BTW, multiple IPv6 addresses on an interface is the norm, and nothing scary.
    – dirkt
    Commented Oct 23, 2017 at 17:57
  • thank you. i understand that i cannot use the link local address to communicate outside my link but this is exactly the point. if the only public address i have is the one on the modem and it is an ipv4 address, how can i communicate outside of my link using ipv6 addresses? i know that there is a class of ipv6 addresses reserved for interoperability with ipv4 but they are not related to the 4 ULAs that i see configured on my pc. so, if i can't communicate outside of my link with ipv6, why do i see these 4 ULAs? and why do i have 4 of them and not just 1?
    – matteof93
    Commented Oct 23, 2017 at 18:05

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If the only public address i have is the one on the modem and it is an ipv4 address, how can i communicate outside of my link using ipv6 addresses?

Because your internet provider does NAT (network address translation) in some way, just like your router does NAT with the 192.168.1.68 private address.

Your router rewrites IPv4 packets coming from this address with a different address (a public IPv4 address, or more likely a provider local address that then again gets NATed by the provider).

In the same way, your internet provider rewrites packets coming from at least one of those three IPv6 addresses, and send them out to the internet with a global IPv6 address. When answering packets arrive, it reverses this process.

If you try to visit some website that lists your public IPv6 address, e.g. this one, you should be able to see the result of the NAT (i.e., some address starting with 2 instead of F).

Why do i have 4 of them and not just 1?

It's completely normal to have multiple IPv6 addresses on one interface. Every interface with IPv6 is required to have a link-local FE80 address, and there's an algorithm that defines which address of those multiple addresses to use as source address.

However, many internet providers just assign a single additional global address.

As to why you have 3 ULA address from your internet provider, I repeat: Only Telecom Italia will be able to tell you, unless someone has figured out the details of the NAT scheme they use. I'm sure they have a reason for it, whatever it may be.

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  • thank you so much for the explanation, i thought that the NAT would only work with IPv4, hence my doubt :) Now that i know that it works also with IPv6 everything is explained.
    – matteof93
    Commented Oct 23, 2017 at 18:33
  • Well, as I also said, IPv6 NAT isn't really necessary, and is considered broken, so I don't understand why they just give you a global address like most other ISPs. But as I don't know any details about what they are doing and why ...
    – dirkt
    Commented Oct 23, 2017 at 18:40

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