Networking – Get IPv6 address of an IPv4 machine

arpipv4ipv6networking

Given two machines on a local network, both connected via IPv4 and IPv6, can machine A get the IPv6 of machine B if it has machine B's IPv4 address?

I presume the ARP cache should be able to provide some indication of the IPv6 address, provided traffic has passed between the two has passed to the computer over IPv6? Answers for both Windows and *nix are welcome.

Best Answer

If the mapping between the IPv4 address and MAC address is found in machine A's ARP cache, then you can guess some of machine B's IPv6 addresses (usually one or two) as follows:

The Link local address, which is fe80::Modified-EUI-64. The modified EUI-64 is derived from the MAC address.
One autoconfigured IPv6 address per subnet on the local link. Like the link local address, the bottom 64 bits of this address are the modified EUI-64.

However, note that the link local address may not be that useful (it can only be used to communicate on-link with machine A) and as for autoconfigured addresses, that function is not always enabled.

If you don't know machine A's MAC address then you can't predict anything about iuts IPv6 address(es) from its IPv4 address(es).

Related Solutions

Networking – the mac-address of multicast (ipv6)

Ethernet has "multicast" MAC addresses as well – any MAC address with the "group" bit set is technically a multicast address; IPv6 uses the prefix 33:33:*, while IPv4 uses 01:00:5e:*. There are other widely-used prefixes, see this Wikipedia table for details.

For IPv6 multicast addresses, the last 32 bits of the IPv6 address are OR'd with 33:33:00:00:00:00. For example:

The "all nodes" address ff02::1 is converted to 33:33:00:00:00:01.
Neighbour solicitations for an example address fe80::4a5d:60ff:fee8:658f are sent to the corresponding Solicited-Node multicast address ff02::1:ffe8:658f, which is converted to Ethernet address 33:33:ff:e8:65:8f.

This is described in RFC 2624 section 7.

Networking – Identify device on local network based on IPv6 address

Thanks to this comment on another Super User answer, I discovered the command:

netsh int ipv6 show neighbors

To my great joy I discovered it tells me the MAC address of devices on the local link, just like arp -a does:

Interface 10: Local Area Connection

Internet Address                              Physical Address   Type
--------------------------------------------  -----------------  -----------
<redacted>
fe80::113d:d91e:e685:943b                     4c-72-b9-d2-b5-5d  Reachable
<redacted>

I was then able to compare the Physical Address to the IPv4 leases on my DHCP server and obtain the NetBIOS name of the offending device. Easy peasy.

I still don't understand why this MAC address is resulting in this particular IPv6 Address. According to this online converter, its link local IPv6 address should be fe80::4e72:b9ff:fed2:b55d. But that's another question...

Best Answer

Related Solutions

Networking – the mac-address of multicast (ipv6)

Networking – Identify device on local network based on IPv6 address

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