What are unicast, multicast and broadcast that we often hear in computer networks?

In the field of computer network communications, data transmission modes are mainly divided into three basic forms: unicast, multicast, and broadcast. These three transmission modes have their own characteristics and are suitable for different network application scenarios. This article will systematically analyze these three transmission modes from the aspects of technical principles, application scenarios, implementation mechanisms, advantages and disadvantages.

1. Unicast communication
Unicast is a one-to-one point-to-point communication mode, where data packets are transmitted from a single source point to a single destination receiving point. In the OSI reference model, unicast transmission occurs at the network layer and is implemented through the IP protocol. The destination address of each packet is unique, ensuring that the message is only delivered to a specific target host.

Unicast communication is mainly implemented through the following elements:


Unique addressing: Each terminal device has a unique network address (such as an IP address)
Routing and forwarding: Network devices determine the next hop address of the packet by querying the routing table
Point-to-point transmission: Data is transmitted from the source to the destination in the network according to the optimal path
Unicast is suitable for scenarios that require one-to-one communication, such as:

HTTP communication between a web browser and a server
Email transmission
Remote login (SSH, Telnet)
File transfer (FTP, SFTP)

2. Multicast communication
Multicast is a one-to-many communication mode that allows data packets to be sent to a specific group of receivers at the same time. The sender only needs to send the data once, and the network device is responsible for copying and forwarding it to all members. Multicast uses a specific IP address range (224.0.0.0-239.255.255.255), which identifies a specific multicast group rather than a single host.

The key technologies of multicast communication include:

Multicast group management: Dynamically join and leave group members through IGMP (IPv4) or MLD (IPv6) protocol
Multicast routing: Use specific multicast routing protocols (such as PIM-SM, PIM-DM) to build a multicast distribution tree
Multicast forwarding: Network devices selectively copy and forward data packets based on the multicast tree

Multicast technology is particularly suitable for the following scenarios:

IPTV and online video conferencing
Software distribution and update
Real-time distribution of financial data
Information synchronization in online games


3. Broadcast communication
Broadcast is a one-to-all communication mode, where packets are sent from a single source point to all hosts in a specific network domain. In IPv4, broadcast is implemented through a special broadcast address, which is usually a combination of a network number and an all-one host number (such as 192.168.1.255). It is worth noting that the IPv6 protocol has cancelled the broadcast mechanism and instead relies more on multicast.

Broadcast is mainly implemented through the following mechanisms:

Broadcast address: Use a specific target address (such as a subnet broadcast address or 255.255.255.255)
Link-layer broadcast: Usually use a MAC-layer broadcast address (FF:FF:FF:FF:FF:FF)
Broadcast domain control: Divide the broadcast domain through network devices (routers) to limit the broadcast range


Broadcasting is usually used in the following scenarios:

Address Resolution Protocol (ARP) request
DHCP service discovery
Routing Information Protocol (RIPv1) update
Network discovery service

IV. Summary
Comparison of three transmission modes:

Transmission methods used by common routing protocols:


Unicast, multicast and broadcast are the three basic modes of network communication. Each has its own characteristics and together they form the foundation of modern network communication. In practical applications, the appropriate transmission mode should be selected according to specific needs.