What is a unicast MAC address and how does the source host determine the destination MAC?
A unicast MAC addresses exactly one destination device. The sender finds the destination MAC for an IPv4 address with ARP, and for an IPv6 address with Neighbor Discovery (ND).
A unicast MAC address is a unique address used when a frame is sent from one device to a single destination device.
Finding the destination MAC:
| For | Protocol Used |
|---|---|
| IPv4 | ARP (Address Resolution Protocol) |
| IPv6 | ND (Neighbor Discovery) |
Key rule: The source MAC address must always be a unicast - you can only send FROM one device.
Context: When Host A wants to send data to Host B's IP address, it first needs to discover Host B's MAC address. ARP/ND maintains a cache mapping IP addresses to MAC addresses.
Go deeper:
-
Address Resolution Protocol (Wikipedia) — how IPv4 hosts discover the MAC behind an IP via broadcast request and unicast reply. -
Neighbor Discovery Protocol (Wikipedia) — the IPv6 equivalent, using solicited-node multicast instead of broadcast.