Unicast vs Multicast

Both are IP delivery modes describing how many receivers a single stream of data reaches. Unicast is one-to-one communication between exactly one sender and one receiver; multicast is one-to-many, where one sender transmits a single copy of the data that is replicated by routers only along the branches leading to subscribed receivers.

1. Comparison table

Basis	Unicast	Multicast
Communication type	One-to-one (single sender, single receiver)	One-to-many (single sender, group of receivers)
Address used	Class A/B/C unicast address (host-specific)	Class D address, range 224.0.0.0 – 239.255.255.255
Address identifies	One specific host interface	A group, not a host — no fixed location
Receiver membership	Implicit — the address names the receiver	Explicit join/leave via IGMP (Internet Group Management Protocol)
Packet replication	One copy sent to that host only	Router replicates packets only on links with group members
Bandwidth usage	$N$ receivers ⇒ $N$ copies of the traffic on the source link path	One copy leaves the source; total network load ≪ $N$ copies
Router behaviour	Standard forwarding along the unicast route	Needs multicast routing (e.g. PIM, DVMRP) and a shared/distribution tree
Routing table	Simple per-destination unicast route	Separate multicast forwarding table built from group state
Use of receiver address	Destination address is the receiver's unicast IP	Destination address is the group address; receiver IP never appears in the header
Example protocols/applications	HTTP, FTP, SMTP, SSH	IPTV, video conferencing, live streaming, OSPF/MOSPF updates, news feeds
Scalability of delivery	Scales with number of receivers (source sends repeatedly)	Scales well — sender effort is constant regardless of group size
Error/ack behaviour	Each receiver can be acknowledged individually	No per-receiver ACK (would defeat the purpose); unreliable, best-effort
Address classes (IPv4)	A, B, C	D only
Requires	Nothing beyond routing	IGMP at host/router, multicast-capable routers and protocols

2. Why multicast is efficient

If a source must send a 1 Mbps video stream to 1000 users:

• Unicast: the source (or its access link) transmits $1000 \times 1 \text{ Mbps} = 1000$ Mbps.
• Multicast: the source transmits 1 Mbps once; the network duplicates packets only at routers where members exist, so link carrying depends on topology, not on group size.

3. Diagram (conceptual)

        Unicast (3 copies)                Multicast (1 copy, replicated)

   S ──► R1 ──► H1                    S ──► R1 ──┬──► H1 (member)
        │                             (one copy) ├──► H2 (member)
        ├──► R2 ──► H2                          │
        │                                       R2 ──► H3 (member)
        └──► R3 ──► H3  (non-member: no copy)

4. Summary

Unicast addresses a device and is used for ordinary request–response traffic where reliability and individual delivery matter. Multicast addresses a group and is used for bandwidth-hungry one-to-many distribution, where sending a single copy and letting routers replicate it saves enormous network capacity — at the cost of needing IGMP plus multicast routing.