THE PROBLEM
An IP packet needs a local link destination before Ethernet can deliver it to the next hop.
THE NAIVE APPROACH
Sending an Ethernet frame to an IP address directly does not work: Ethernet needs a MAC destination.
THE KEY INSIGHT
A host broadcasts a request asking who owns an IPv4 address; the owner replies with its hardware address.
WHERE YOU MEET IT
- • A laptop reaching another device on the same LAN
- • A host finding its default gateway MAC
- • A printer responding to a workstation
ANALOGY / HISTORY
Like asking a building receptionist which desk belongs to a person.
Context: ARP filled the IPv4-to-Ethernet address-resolution role on local networks.
client → server · REQUEST BROADCAST
ARP request
The requester broadcasts an Ethernet/IPv4 address-resolution request. The target hardware address is unknown and set to zero.
ARP resolves only local-link neighbors. For a remote destination, a host normally resolves its default gateway instead.
ARP for Ethernet/IPv4 is 28 bytes: hardware/protocol types and lengths, opcode, sender MAC/IP and target MAC/IP.