Unicast, Multicast and Broadcast Communication

Devices in the network can send data over the network using some communication techniques known as Unicast, Multicast and Broadcast. Unicast is the process of sending a data packet from one host to an individual host. The Broadcast technique send a data packet from one host to all hosts in the network. In 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 route through an internetwork. There is just one sender, and one receiver in unicast communication.

The addresses assigned to two end devices use as the source and destination address in IPv4 unicast communication. The sending host device encapsulates its own 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 reserved for special purposes.  Remember that the source address of any packet is always the unicast address of the originating host. The figure below illustrates the unicast communication.

Unicast

Multicast

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

A router connected to the local network knows about the multicast packets 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, 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.

Multicast

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

224.0.0.0 Base address
224.0.0.1 Reserved for all hosts on the same network segment.
224.0.0.2 Reserved for all Routers multicast group addresses on the same network segment.
224.0.0.4 Distance Vector Multicast Routing Protocol (DVMRP) multicast routers address.
224.0.0.5 Reserved for all OSPF Routers for sending Hello packets to all OSPF routers on a network segment.
224.0.0.6 Reserved for sending OSPF routing information to designated routers (DR) on a network segment.
224.0.0.9 The RIP version 2 group address used to send routing information to all RIP2 routers on a network segment.
224.0.0.10 Reserved for EIGRP group address to send routing information to all EIGRP routers on a network segment.
224.0.0.18 Reserved for Virtual Router Redundancy Protocol (VRRP)
224.0.0.19–21 Reserved for IS-IS over IP
224.0.0.102 Reserved for Hot Standby Router Protocol version 2 (HSRPv2) and Gateway Load Balancing Protocol (GLBP)
224.0.0.251 Reserved for Multicast DNS address
224.0.0.252 Reserved for Link-local Multicast Name Resolution address
224.0.1.1 Reserved for Network Time Protocol clients. The clients listen on this address for protocol messages when operating in multicast mode.
224.0.1.41 H.323 Gatekeeper address

Broadcast

In the broadcast communication, one host is sending a packet to all other hosts over the network. There is only one sender in the broadcast process, but the data is receiving to 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 that the whole local network will receive the packet.

Network protocols also used 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 the Wi-Fi networks which declare themselves to all nearby wireless devices using a broadcast technique. Wi-fi broadcast their SSID to makes it easier for users to find a nearby network.

The broadcast has two types, directed and limited. A directed broadcast sent 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 traffic receiving 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 to 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 so that it 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 performance of the network. The figure below illustrates the broadcast communication.

Broadcast