Map the next local IP address to a MAC address
04Before Ethernet can deliver a frame, the sender needs a local destination MAC address. A static table or ARP can provide the mapping for an IP next hop.
Separate /24 example: host bits all ones make 192.0.2.255 the local subnet broadcast. Map it directly to FF:FF:FF:FF:FF:FF; the drawing shows local copies, not router forwarding.
Source for this drawing: RFC 894 · Read RFC source: Address Mappings (unnumbered) ↗
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.
For an off-link destination, resolve the local router’s address, while preserving the packet’s destination IP.
- RFC 894 permits a static address table or dynamic discovery and strongly recommends ARP. It does not require pretending that a static table is impossible.
- A host must know its own Internet address and respond appropriately to ARP. RFC 826 explains a request broadcast and the target’s reply used to discover a local mapping.
- For an off-link destination, route selection chooses the local next hop first. The Ethernet destination is the router’s MAC; the IP destination still identifies the remote host.
- The subnet broadcast IP address (host bits all ones) should map to FF:FF:FF:FF:FF:FF. The example 192.0.2.255 is for a chosen /24 subnet, not a rule that every address ending in 255 is broadcast.
All hosts and addresses are fictional. The broadcast example is confined to this local link; it is not permission to forward directed broadcasts between networks (see RFC 2644). The historical all-zero-host-part 4.2BSD behavior in RFC 894 is not taught as the standard rule. ARP supplies a mapping, not cryptographic proof of identity.
Read the RFC section ↗