Wanted to write this up in case someone else is scratching their head with the same issue.
Some routers are able to use both 2.4 GHz and 5 GHz frequencies, allowing devices to appear as though they are on the same network regardless of which they use. These actually provide two different wireless networks, with different SSIDs, and bridge between the two.
This works fine for most things, but today I realised that it’s the cause of a printer problem I’d been having. The printer was using the 2.4 GHz network while my laptop was using the 5 GHz one. When I tried to print, the mDNS name of the printer was needed so a multicast query was sent… but the printer didn’t respond.
The reason is the printer never received the query. The mDNS multicast address is 126.96.36.199, which is for multicast to the local subnet only. My router interprets this to mean that packets sent on the 2.4 GHz network will not reach the 5 GHz network.