Unicast, Multicast and Broadcast Communication

Devices in the network can send data over the network using communication techniques known as Unicast, Multicast and Broadcast. Unicast is sending a data packet from one host to an individual host. The Broadcast technique sends a data packet from one host to all hosts in the network. In the Multicast technique, a packet is sent from one host to a group of hosts, not to all hosts, possibly in different networks. In all cases, the packet header must contain the originating host address as the source address.

Unicast

In the unicast technique, communication occurs between host-to-host over the network. The packet in unicast transmission contains the destination device address as a destination address and can be routed through Internetwork. There is just one sender and one receiver in unicast communication.

The addresses assigned to two end devices are the source and destination addresses in IPv4 unicast communication. The sending host device encapsulates its IPv4 address as a source host address and the destination host address as the destination address during the encapsulation process. The range of IPv4 unicast addresses is from 0.0.0.0 to 223.255.255.255, but inside this range, many addresses are reserved for special purposes.  Remember that any packet’s source address is always the originating host’s unicast address. The figure below illustrates the unicast communication.

Multicast

Multicast is another communication technique used in networking. In multicasting, the data packet is sent from one or more hosts to a group of hosts. There are possibly one or more senders, and the information is distributed to a group of receivers. It reduces traffic by allowing a host to send a single packet to a selected set of hosts that are multicast group members.  The IPv4 address range 224.0.0.0 to 239.255.255.255 is reserved for multicasting.

A router connected to the local network knows about the multicast packet destinations. So, the router forwards the packets to the multicast group and never forwards them further. A typical use of the reserved multicast address is in routing protocols is to exchange routing information. For example, 224.0.0.5 and 224.0.0.6 are reserved for OSPF, and 224.0.0.9 is reserved for RIP version 2. The figure below illustrates the multicast communication.

The multicast client can receive multicast data using client program services. Each multicast group is its own IPv4 multicast destination address. When a host subscribes to a multicast group, the host processes packets addressed to this multicast address and packets to its uniquely allocated unicast address. Some important reserved multicast addresses are the following:-

Broadcast

In broadcast communication, one host sends a packet to all other hosts over the network. There is only one sender in the broadcast process, but the data is received by all other connected hosts in the network. The source sends data to all hosts in the network using a broadcast address. The broadcast packet contains the destination IPv4 address with all ones in the host portion. All 1s in the host portion means the whole local network will receive the packet.

Network protocols also use the broadcast technique, such as DHCP. When a host receives a packet sent to the network broadcast address, the host processes this packet as it would a packet received to its unicast address. Another example of the broadcast is Wi-Fi networks, which declare themselves to all nearby wireless devices using a broadcast technique. Wi-Fi broadcasts their SSID to make it easier for users to find a nearby network.

There are two types of broadcast: directed and limited. A directed broadcast sends traffic to all hosts on a particular network. For example, a host on the 200.100.50.0/24 network sends a packet to 200.100.50.255. This is a routable address, so a router would forward it to the end destination gateway if the router is configured to do so. But a limited broadcast is sent to 255.255.255.255.  The router will send the traffic received to this address to all other hosts on the local network.  255.255.255.255 is not a routable address, so a router would not route the traffic outside the local network. By default, routers do not forward broadcasts.

Broadcast traffic takes network resources because each host on the network must process the broadcast packet. So, broadcasting affects the network and devices’ efficiency. Therefore, broadcast traffic is limited and does not affect the performance of the network and network devices. Routers discourage broadcasting by default, because each port of the router has separate broadcast domains, subdividing of networks also improve the network’s performance. The figure below illustrates the broadcast communication.