Multicast DNS (mDNS) is a protocol aimed at helping with name resolution in smaller networks. In doing so, it takes a different approach than the well-known DNS. Instead of querying a name server, all participants in the network are directly addressed. The appropriate client sends a multicast into the network while asking which network participant matches up with the host name. A multicast is a unique form of communication through which an individual message is directed at a group of recipients. The group can be made up of, for example, the entire network or a sub-network.
In this way, the request also goes to the group participant who owns the host name that is being searched for. The latter responds to the entire network (also via multicast). All participants are informed of the connection between the name and IP address, and can make a corresponding entry in their mDNS cache. As long as this notation is valid, no one in the network needs to request the host name.
The multicast DNS creates a relatively large amount of traffic, yet attempts to save active network resources. For this purpose, the client making the request sends (according to the current cache entry) the reply that, in their opinion, is correct. Only when this is no longer correct, or when the entry is about to expire, does the recipient need to reply. The other participants are already informed before they receive a reply. With this protocol, the traffic within the network can thus be reduced.
In general, only host names with the ending .local are possible with multicast DNS. That limits this form of name resolution on local networks. Host names with other top-level domains (TLD) – such as .de or .com – are not processed by mDNS. Web addresses can, therefore, not be resolved in this way.