Understanding Unicast, Multicast, Broadcast, and Anycast in IP Networks

Unicast

In unicast communication, a single sender transmits data to a single, uniquely identified receiver. In IPv4, addresses from 0.0.0.0 to 223.255.255.255 are unicast addresses. Example: calling a specific person's name, only that person replies.

Multicast

Multicast uses a single multicast address to deliver data simultaneously to multiple receivers on a TCP/IP network, often together with RTP for audio/video. Destination addresses belong to Class D (224.0.0.0–239.255.255.255), which cannot appear as a source address. Example: shouting “beauty” on the street, several women turn around.

Broadcast

Broadcast is not supported by all network types (e.g., X.25, Frame Relay) and does not exist across the entire Internet; IPv6 replaces broadcast with anycast. Broadcast is usually confined to LANs such as Ethernet or token‑ring. Ethernet and IPv4 use the all‑ones address (ff:ff:ff:ff:ff:ff and 255.255.255.255). Token‑ring uses a special value in the IEEE 802.2 control field. Example: shouting “holiday” in an office, all colleagues respond.

Anycast

Anycast differs from the other three methods. An anycast address maps to a group of nodes, but at any moment only one node receives the packet. The network selects the most suitable host based on routing conditions; the chosen host replies with a unicast packet. In the Internet, Border Gateway Protocol (BGP) is commonly used to implement anycast.