Unicast
Unicast is one-to-one communication where a packet is sent from one source to one specific destination. Most internet traffic is unicast – HTTP requests, SSH sessions, email delivery. Unicast requires the sender to know the destination IP address. Contrast with multicast (one-to-many) and broadcast (one-to-all).
Definition
Unicast addresses a single recipient. The source host constructs a packet with one destination IP and the network delivers it exclusively to that host. Every TCP connection is unicast by nature – the 5-tuple identifies exactly two endpoints. Unicast is the default and most common transmission mode. When a server needs to send the same data to 1000 clients (live video), unicast requires 1000 separate streams – each consuming bandwidth on the server's uplink. Multicast solves this by allowing the network to replicate packets at branch points, but multicast requires router support (PIM, IGMP) that is unavailable on the public internet. CDNs solve the unicast scaling problem by placing edge servers close to users, so the origin only sends one copy per PoP.
Examples
- –Any TCP connection is unicast by definition
- –curl http://example.com – unicast HTTP request to one server
- –ping 8.8.8.8 – unicast ICMP to one destination
Related Protocols
Related Terms
Anycast
Anycast assigns the same IP address to multiple servers in different locations. Routers direct traffic to the nearest instance based on BGP path selection. CDNs, DNS root servers, and DDoS mitigation services use anycast to serve users from the closest point of presence – reducing latency and distributing load geographically.
Multicast
Multicast is one-to-many communication where a single packet is delivered to all members of a group simultaneously. The network replicates packets at branch points rather than the sender duplicating them. Used for IPTV, stock market feeds, and cluster discovery. Requires IGMP (hosts) and PIM (routers).
Broadcast
Broadcast sends a packet to all hosts on a network segment simultaneously. The IPv4 broadcast address (e.g., 192.168.1.255 for /24) delivers to every host on the subnet. ARP, DHCP discovery, and NetBIOS use broadcast. Excessive broadcasts cause broadcast storms. IPv6 eliminated broadcast entirely in favor of multicast.