[jeroenhd]

It does! I generally assume mDNS to just be available on every device these days. But I've also seen managed environments where mDNS has been turned off or blocked at the firewall.

[dboreham]

mDNS is a broadcast protocol so always "blocked at the firewall ".

[oasisbob]

Multicast too. If you've never needed to manipulate ACLs for multicast traffic, you're not really living.