Distinguish an ARP Probe from an Announcement
01Address Resolution Protocol (ARP) connects an IPv4 address with a local hardware address. Address Conflict Detection (ACD) uses the same packet format to ask whether an address is occupied before claiming it.
Separate format comparison: an Announcement retains ARP Request opcode1 but changes sender IP to 192.0.2.10, equal to target IP. Actual announcing follows the probe sequence in later chapters.
Source for this drawing: RFC 5227 · Read RFC section 1.1 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
RFC 5227 formally updates RFC 826 by specifying when to generate probes and how to detect conflicts. It does not change the ARP packet format or existing reception rules.
Read the updated RFC section ↗A Probe has sender IP zero; an Announcement puts the claimed IP in both IP fields.
- A Probe is an ARP Request, broadcast on the local link. Sender hardware MUST be the sending interface address; sender IP MUST be all zeroes; target IP MUST be the candidate.
- The ARP target hardware field is ignored and SHOULD be zero. It is different from the outer Ethernet destination, which is broadcast in this example.
- An Announcement is also a broadcast ARP Request, with sender and target IP both the announced address. It is not an ARP Reply or proof of exclusive ownership.
The drawings use fictional documentation IPv4 address 192.0.2.10 and fictional hardware identifiers. Field widths are schematic. Format comparison is not permission to announce before successful probing. ACD is limited to a shared link, not duplicate addresses on unrelated networks (§1.3).
Read the RFC section ↗