What is Unicast?
Unicast is one-to-one packet delivery from a single source to a single destination, the most common form of network communication on the Internet.
Unicast is a method of network communication in which data is transmitted from one sender to one specific receiver. Each unicast packet carries a destination address that identifies a single interface on the network. This one-to-one delivery model is the foundation of most Internet traffic, including web browsing, email, file transfers, and remote login sessions.
In IP networking, unicast is implemented using either IPv4 or IPv6 unicast addresses. An IPv4 unicast address belongs to classes A, B, or C (or is assigned from the global unicast space under CIDR). An IPv6 unicast address is identified by the address type prefix (for example, 2000::/3 for global unicast). When a host sends a unicast packet, switches and routers forward it hop by hop toward the destination, using the destination address to determine the next hop. The packet is delivered only to the intended recipient; no other host on the network processes it.
Unicast contrasts with multicast (one-to-many) and broadcast (one-to-all). It is the default and most widely used delivery method in TCP/IP networks. Protocols such as ARP, DNS, HTTP, and SSH all rely on unicast communication. While unicast is simple and reliable, it does not scale well for one-to-many distribution because the sender must replicate packets for each receiver, consuming bandwidth and processing resources.
Key facts
- Unicast delivers packets from one source to exactly one destination interface.
- IPv4 unicast addresses include Class A, B, C, and CIDR-assigned global addresses.
- IPv6 unicast addresses use prefixes like 2000::/3 for global unicast.
- Unicast is the default delivery mode for TCP, UDP, and most application protocols.
- Unlike multicast, unicast requires the sender to send a separate copy of each packet per receiver.
How it works in practice
Related terms
References
More in Networking & Routing
Anycast
Anycast is a network addressing and routing method where a single IP address is assigned to multiple servers, and routers send traffic to the nearest server based on routing protocol metrics.
AS Path
A BGP path attribute that lists the sequence of autonomous system numbers a route has passed through, used for loop detection and path selection.
ASN
A globally unique 16 or 32 bit number assigned to an autonomous system for use in BGP routing between organizations on the Internet.
Autonomous System
An Autonomous System (AS) is a group of IP networks under a single administrative routing policy, identified by a unique ASN (Autonomous System Number) for exterior gateway routing.
BGP
BGP (Border Gateway Protocol) is the path vector routing protocol that networks use to exchange reachability information between autonomous systems on the public internet.
CIDR
CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing packets using variable-length subnet masks (e.g., /24) instead of fixed classful boundaries.
Hop
A hop is one passage of a packet through a router or other layer-3 forwarding device as it travels from source to destination across an internetwork.
IPv4
IPv4 is the core Internet Protocol using 32-bit addresses, providing roughly 4.3 billion unique identifiers for network interfaces on the global internet.
IPv6
IPv6 is the most recent version of the Internet Protocol, using 128-bit addresses to provide an effectively unlimited number of unique identifiers for networked devices.
Latency
Latency (or round-trip time, RTT) is the time required for a packet to travel from a source to a destination and back, measured in milliseconds, and is a critical metric in network performance.