An ARP reply is a unicast frame that tells the device that sent an ARP request which MAC address belongs to the requested IP address. ARP lets hosts, routers, and other devices learn the MAC address needed to deliver a frame on the local network. section 6.4 section 6.6
The exchange works as follows:
- A device that lacks a destination MAC address sends an ARP request to the broadcast MAC address ffff.ffff.ffff. section 6.4
- Switches flood the request through the broadcast domain. Hosts whose IP address does not match the request ignore it. section 6.4 section 6.6
- The host owning the requested IP address sends an ARP reply directly to the requester’s MAC address. section 6.4
- Both devices can add IP-to-MAC mappings to their ARP tables, avoiding a new request before every packet. section 10.1.1 section 10.3
Learning a default gateway’s MAC address
PC1 knows that its default gateway is 192.168.1.1 but does not know its MAC address. PC1 broadcasts an ARP request for that IP address. R1 replies unicast, and PC1 stores the mapping for R1’s G0/1 interface. PC1 can then place the original packet in a frame addressed to R1. section 10.1.1
Resolving a routed next hop
R1’s routing table identifies 192.168.12.2 as the next hop toward 192.168.3.0/24. R1 uses ARP to learn R2’s MAC address, receives an ARP reply, and encapsulates the packet for R2’s interface. The same process lets R3 resolve PC3’s MAC address on its directly connected network. section 10.1.2 section 10.1.4
