Difficulty: Easy
Correct Answer: Multicast is one-to-many communication where data is sent to a specific group of interested receivers that have joined a multicast group address, instead of all hosts in the network.
Explanation:
Introduction / Context:
Multicast is an important concept in IP networking, used for applications like IPTV, video conferencing, streaming media, and routing protocols. Unlike broadcast, which floods traffic to every host in a broadcast domain, multicast is designed to deliver traffic only to hosts that explicitly request it. Interviewers often ask candidates to differentiate multicast from broadcast and unicast and to explain its basic purpose.
Given Data / Assumptions:
Concept / Approach:
Multicast communication is one-to-many or many-to-many, but only for a defined group. A sender transmits packets to a multicast group IP address, and only receivers that have joined that group receive the traffic. Network devices such as switches and routers learn which ports or subnets have group members and forward multicast packets accordingly, often using IGMP snooping on switches and multicast routing protocols like PIM on routers. This selective delivery is more efficient and scalable than broadcast, which blindly sends traffic to every host in the broadcast domain, whether they want it or not.
Step-by-Step Solution:
Step 1: Define multicast as communication from one sender to multiple receivers that have explicitly joined a multicast group.
Step 2: Explain that the sender uses a multicast IP address, not a list of individual unicast addresses.
Step 3: Describe how group membership is signalled by hosts using IGMP, allowing network devices to know where to forward multicast packets.
Step 4: Contrast this with broadcast, where packets are sent to a broadcast address and delivered to every host in the local broadcast domain.
Step 5: Emphasize that multicast aims to be more bandwidth efficient by sending a single stream that the network replicates only where needed.
Verification / Alternative check:
In a test lab, if one host joins a multicast group and another host on the same subnet does not, packet capture shows that multicast traffic is delivered only to the subscribed host (assuming IGMP snooping is active), while the other host may not see those packets. Comparing this to a broadcast such as an ARP request, all hosts on the subnet see the broadcast regardless of interest. This behaviour verifies the selective nature of multicast.
Why Other Options Are Wrong:
Option B describes unicast communication, which is strictly one-to-one. Option C exaggerates by claiming multicast must reach every device on the internet; in reality, multicast is specifically scoped and controlled. Option D is wrong because multicast is very much used by end hosts, for example in streaming media clients, not just by routers.
Common Pitfalls:
A common misconception is to treat multicast like broadcast and assume it will automatically reach every host. Another pitfall is forgetting that multicast requires proper configuration of IGMP and multicast routing; without it, traffic may be flooded inefficiently or blocked entirely. Understanding that multicast is group based, opt-in delivery helps in designing scalable multimedia and collaboration systems.
Final Answer:
Multicast is one-to-many communication where traffic is sent to a defined group address and delivered only to receivers that have joined that group, instead of all hosts like a broadcast.
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