IPv4 Communication – Unicast, Broadcast and Multicast

There are three types of communication process used for sending data over networks. In all cases, the packet header must contain the originating host address as source address. All three communication process is the following:-

  • Unicast– The process of sending a data packet from one host to an individual host.
  • Broadcast– The process of sending a data packet from one host to all hosts in the network.
  • Multicast– The process of sending a packet from one host to a group of hosts, not to all hosts,  possibly in different networks

Unicast Transmission

Unicast is the communication process between host-to-host over a network. the unicast packets used the destination device address as a destination address and can route through an internetwork. In unicast transmission, there is just one sender, and one receiver.

In an IPv4 network, for unicast communication, the addresses assigned to the two end devices use as the source address and destination address. The source host encapsulates its own IPv4 address as source host address and the IPv4 address of the destination host as the destination address during the encapsulation process. Remember that the source address of any packet is always the unicast address of the originating host.

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.  We will discuss these special purposes in coming articles.

Broadcast Transmission

The broadcast is a communication term where one host is sending a packet to all other hosts over the network. In this case, there is just one sender, but the information is receiving to all connected hosts.

The broadcast sends packets to all hosts in the network using a broadcast address. With a broadcast, the packet header contains a destination IPv4 address with all ones (1s) in the host portion. This means that all hosts on that local network will receive the packet. Many network protocols used to broadcast, 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.

There are two types of a broadcast: directed and limited. A directed broadcast is sent to all hosts on a particular network. For example, a host on the 172.16.16.0/24 network sends a packet to 172.16.16.255. This address is routable 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.  It is not a routable address, so a router would not route it anywhere. By default, routers do not forward broadcasts.

A broadcast packet uses resources on the network and every host on the network must process the packet. So that affects the efficiency of the network and its devices. Therefore, broadcast traffic is limited so that it does not affect the performance of the network or devices. Because routers separate broadcast domains, subdividing of networks can also improve the performance of the network.

Multicast Transmission

Multicast is a communication term where a data packet is sending from one or more host to a group of other hosts. In this case, there is possibly one or more senders; and the information is distributed to a group of receivers. Multicast transmission reduces the traffic by allowing a host to send a single packet to a selected set of hosts that subscribe to a multicast group.

IPv4 has reserved the address range of 224.0.0.0 to 239.255.255.255 for multicasting. The 224.0.0.0 to 224.0.0.255 are IPv4 multicast addresses on the local network only. A router connected to the local network know that these packets are addressed to a local network multicast group and never forwards them further. A typical use of reserved local network multicast address is in routing protocols using the multicast transmission to exchange routing information. For example, 224.0.0.9 is the multicast address uses by RIP version 2 to communicate with other RIP versions 2 routers.

The multicast client can receive multicast data by using client program services. The client program is used to subscribe to the multicast group. 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.

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