RFC 826 / START HERE
ARP, before the details
ARP: Address Resolution Protocol
ARP helps an IPv4 device find the local link-layer address associated with an IPv4 address on its local network.
The idea in one picture
A device knows a local IP address
ARP asks who has it
A link-layer address is learned
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
An IP destination alone is not enough to send a frame on an Ethernet link. The sender needs the local next hop's hardware address too.
A familiar example
Before sending traffic to your home router, a computer asks which local device has the router's IPv4 address.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Resolve the next hop on a linkRouting first selects a local next hop; ARP then looks up that address on the selected link.→
- Broadcast a question and receive a replyAn Ethernet ARP request asks which station owns a target protocol address.→
- Read the variable-address packetARP identifies hardware and protocol spaces, address lengths, opcode, and sender and target address pairs.→
- Learn from the sender mappingThe original receive algorithm merges sender information into an existing table before examining the request opcode.→
- Probe before claiming an IPv4 addressLater conflict-detection rules add deliberate probes and announcements using ARP.→
This introduction explains RFC 826 (1982). Read the original RFC ↗