alt Hacker NewsI 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).
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mDNS on link-locals is what makes the "plug computers and printers into switch" case work. It would have been NetBIOS originally but mDNS is how it's done today.
Link-local addresses (LLAs) are needed in IPv6 because IPv6 doesn't have broadcast. IPv6 uses multicast instead.
Broadcasts go to all IPv4 addresses in the subnet, multicasts only go to those who subscribed to a multicast group. To subscribe to a IPv6 multicast group you need an IPv6 address. So all IPv6 interfaces will have at least one LLA self-generated.
One thing that IPv6 uses multicast heavily for is NDP, which is the IPv6 version of ARP. This is how IP addresses on your LAN/WLAN are converted to MAC addresses which is required info for the NIC in your node to talk to another node on your Ethernet LAN/WLAN.
End users don't typically have to use LLAs directly but you can use them if you want to 100% ensure things won't leave your LAN as routers don't forward LLAs.