Ethernet, Switching & ARP
Explain local forwarding and how IPv4 finds a neighbor's link address.
Frames and learning switches
Ethernet frames include source and destination MAC addresses. A switch learns the incoming port for each source MAC, then forwards a known destination only through its learned port. An unknown destination can be flooded through other ports within that local broadcast domain.
A switch forwarding a frame on one LAN is different from a router moving an IP packet between subnets.
ARP and the first hop
On IPv4 Ethernet, a host uses ARP to discover the MAC address for an on-link next hop. If the destination is outside its subnet, the host resolves the default gateway's MAC, not the remote server's MAC. The local frame is then sent to the gateway, which chooses the next hop.
ARP mappings are temporary and apply to the local link; IPv6 uses Neighbor Discovery rather than ARP.
Check your understanding
When reaching a remote subnet, whose MAC address does the sending host need first?