mDNS
Also called: Bonjour, zeroconf, avahi, .local
mDNS lets devices on the same network find each other by name without a DNS server, which is how a freshly booted appliance can be reached at a .local address.
mDNS (multicast DNS) resolves hostnames on a local network with no DNS server. A
device claims a name in the .local domain and answers multicast queries for it.
This is what makes first-boot setup possible without a screen. Flash an image, plug in the
network, and the device is reachable at something like deputyos.local — no need to hunt
through the router’s DHCP table for an address.
It is implemented by Avahi on Linux, Bonjour on macOS and iOS, and natively on Windows 10 and later. Support is essentially universal on consumer networks.
The limits are worth knowing: mDNS does not cross subnets, and many enterprise and guest networks block multicast. Where it fails, the fallback is the IP address.