ARP — Requests, Caches & Gratuitous

ARP maps IPv4 → MAC inside one broadcast domain: request is a broadcast “who has 192.168.1.20?”, the reply is a unicast. Stateless, unauthenticated, and the first packet pays the cost — everything else hits the ARP cache (~4 h aging on Cisco). The exam-grade details Routers ARP for the next-hop IP, never the final destination. […]