alt Hacker NewsIt’s not exclusively an IPv6 feature: RFC 3927 defines link-local IPv4 addresses, to be assigned randomly from 169.254.0.0/16 after a bit of ceremony to detect collisions.
Ideally, you’d be able to connect a PC and a printer with an Ethernet cable, they would both (having failed to find a better alternative) allocate a link-local address for themselves, and then the PC would use DNS-SD over mDNS to discover the printer and show it to you. Similar story with PCs exporting their media files over the network, a—say—set-top box, and a switch they’re all plugged into.
And for some combinations of parts this actually works. It’s just that the functionality is not always well-exposed by the OS, that a switch + DHCP server in a box (in practice, a consumer router) can work just as well with no configuration as an unmanaged switch can, and that people are not that interested in local-only wired networks anymore.
There’s also the “having failed to find a better alternative” part: unlike with IPv6, the RFC does not endorse always assigning a link-local address as the second one next to a static or DHCP-provided one, I’m guessing for software compatibility. Thus you really only see 169.254.* in your interface configuration when DHCP is borked, and it’s kind of useless in that case.
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> to be assigned randomly from 169.254.0.0/16
Yes, but the question is, "what if an address in this range is assigned to _two_ interfaces at the same time?" Now your local routing information base cannot distinguish which interface to use when trying to reach other hosts in that same network. So, it's fair to say, it's not a feature even available in IPv4.
The second difference is IPv6 is almost always going to have link local addresses assigned and machines with multiple network interfaces are the norm rather than the exception.
I literally had to interrogate an LLM to explain what this was about, because to me, indeed, when I see 169.254 I think "Ah, someone unplugged something critical and the network is now completely down." I didn't even know that in ipv6 land there are any reasons to use link-local addresses for anything. I mean, there still basically isn't a reason for 99.99% of people, I think. But it's interesting.
I also didn't realize that part of the idea behind these LL things was one of the rounds of wishful networking ideas of the 90s or 2000s, kind of a cousin of UPnP and mDNS in that way (in increasing order of eventual usefulness).
Considered completely in a vacuum, especially ignoring the WAN, I can see how it seemed silly that if you plugged three computers and a printer into a switch, rolling random IP addresses like this could have allowed things to be discoverable and to function locally (I thought mDNS or "Bonjour"/"Rendezvous" as Apple called it came much later, but I know my PCs could "see" each other with NetBIOS or whatever long before mDNS was invented).
The IPv6 feature isn’t link-local addresses, it’s being able to specify the interface to bind to as part of the address specification. This lets you demand that your IPv6-based tool use your wired Ethernet connection, for example.