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Networking Fundamentals · Intermediate

Multicast DNS (mDNS)

A zero-configuration protocol that resolves .local names on the local link using multicast queries to UDP port 5353.

Definition

mDNS (RFC 6762) lets devices such as printers, speakers and Apple devices find each other without a DNS server, and DNS-SD layers service discovery on top of it. Its traffic does not cross routers without a gateway or reflector, which is why printers on another VLAN may not be discoverable. Windows also uses the older LLMNR protocol for similar local resolution, which security guidance recommends disabling.

Example

A Mac resolves printer.local by sending an mDNS query to 224.0.0.251:5353.

Exam tip

mDNS = UDP 5353 and stays on the local link; cross-VLAN discovery needs an mDNS gateway.

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