Unicast

Unicast as the Default Model

The vast majority of internet traffic is unicast: HTTP/HTTPS page loads, API calls, email delivery, SSH sessions, video calls. Each TCP connection is unicast — a socket pair binding one source IP:port to one destination IP:port. UDP is also predominantly unicast for DNS queries, HTTP/3 (QUIC), and most application protocols.

Unicast's simplicity is its strength. No group membership management is required (as with MulticastA network communication method that sends data from one source to a specific group of interested receivers simultaneously. Uses the 224.0.0.0/4 range in IPv4 and is commonly used for streaming and routing protocols.), no broadcast domain constraints apply (as with BroadcastA network communication method that sends data to all devices on a subnet simultaneously. The broadcast address is the last address in a subnet, such as 192.168.1.255 for a /24 network.), and no topology-aware routing tricks are needed (as with AnycastA network addressing method where the same IP address is assigned to multiple servers in different locations. Traffic is routed to the nearest or best-performing server, commonly used by CDNs and DNS providers.). A packet travels from source to destination following the routing table's longest-prefix match.

Unicast Address Scopes

Unicast addresses exist at multiple scopes. Link-local unicast addresses (169.254.0.0/16 in IPv4Internet Protocol version 4. The fourth revision of IP using 32-bit addresses (e.g., 192.168.1.1), providing approximately 4.3 billion unique addresses. Still the most widely used internet protocol despite address exhaustion., fe80::/10 in IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion.) are valid only on a single network segment and are used for neighbor discovery and DHCP alternatives. Globally unique addresses (public IPv4Internet Protocol version 4. The fourth revision of IP using 32-bit addresses (e.g., 192.168.1.1), providing approximately 4.3 billion unique addresses. Still the most widely used internet protocol despite address exhaustion. or IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion. global unicast) route across the entire internet. Private unicast addresses (Private IP AddressAn IP address from reserved ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) used within local networks. Private addresses are not routable on the public internet and require NAT for external communication.) route within organizational networks but not on the public internet.

TCP and Unicast Reliability

Unicast underpins TCP's reliable, ordered delivery. TCP's three-way handshake (SYN, SYN-ACK, ACK), acknowledgment numbering, retransmission timers, and congestion control all assume a dedicated bidirectional channel between exactly two endpoints. These mechanisms would be fundamentally impossible with BroadcastA network communication method that sends data to all devices on a subnet simultaneously. The broadcast address is the last address in a subnet, such as 192.168.1.255 for a /24 network. or MulticastA network communication method that sends data from one source to a specific group of interested receivers simultaneously. Uses the 224.0.0.0/4 range in IPv4 and is commonly used for streaming and routing protocols., where packets go to multiple receivers.

Use Ping Test to verify unicast reachability between your host and a specific destination. Use Open Port Checker to test whether a unicast TCP connection to a specific port succeeds.