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unicast multicast broadcast

Unicast

Unicast, Multicast and Broadcast Communication

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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.

Unicast

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.

Multicast

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:-

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

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.

Broadcast
Unicast, Multicast, and Broadcast Addresses

Unicast, Multicast, and Broadcast Addresses

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In Ethernet, broadcast and multicast addresses always describe a group of recipients. However, the unicast explains communication where data is sent from one point to another.

Unicast Address

In the case of a unicast address, a packet has been sent from a single source to a specific destination. All Ethernet and IP networks support unicast transmission. A unicast MAC address is the unique address used to send a packet from a single transmitting device to a single destination device.

A destination IP address must be in the IP packet header to send a unicast packet from source to destination. A destination MAC address must also be present in the Ethernet frame header. The IP and MAC addresses combine to deliver data to a specific destination.

Multicast Address

A multicast address is a logical identifier for a group of hosts in a computer network that is available to process datagrams or frames intended for multicasting for a designated network service. The Data link layer of the OSI model also uses a multicast address.

The multicast MAC address is a special value that begins with 01-00-5E in hexadecimal. The range of multicast MAC addresses is from 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF for Ethernet and Fiber Distributed Data Interface (FDDI) media access control (MAC) addresses. The remaining portion of the multicast MAC address is created by converting the lower 23 bits of the IP multicast group address into 6 hexadecimal characters.

The range of IPv4 multicast addresses is 224.0.0.0 to 239.255.255.255. With the help of multicast addresses, a source device can send a packet to a group of devices that belong to a multicast group. These devices assign a multicast group of IP addresses. The source will also be a unicast address.

Online games also use multicast addresses, where many players connect remotely but play the same game. Another use of the multicast address is in distance learning through video conferencing, where many students connect to the same class.

Broadcast Address

When a single computer sends messages to many at once, it is called a broadcast. Television broadcasting is the most common example of a broadcast. Radio transmission is another example of broadcasting.

In computer networking, a broadcast means the network sends one copy of a packet to each destination. The terms broadcast IP address and broadcast MAC address are common in networking.

A broadcast packet also contains a destination IPv4 address with all (1s) in the host portion (see 172.17.255.255) in the IP Packet. This numbering in the address means that all hosts on that local network (broadcast domain) will receive the packet.

Several network protocols, such as ARP and DHCP, use broadcasts. When the IPv4 broadcast packet is encapsulated in the Ethernet frame, the destination MAC address is the broadcast MAC address of FF-FF-FF-FF-FF-FF in hexadecimal, which is 48 ones in binary.