RFC 894 / START HERE
IP over Ethernet, before the details
IP means Internet Protocol; IPv4 is version 4. Ethernet is the name of a local-network technology. RFC 894 explains how an IPv4 datagram travels inside a standard Ethernet frame.
The IP datagram contains its own header and data. Ethernet adds local Media Access Control (MAC) addresses and a type field: hexadecimal 0800 identifies IPv4. A short datagram needs link padding, but those extra bytes do not become part of the IP packet.
Why it exists
The publication established an interoperable framing method for the 10-megabit Ethernet of its time. It is an April 1984 Internet Standard with no formal update or obsoletion edges in the fetched RFC Editor metadata. Verified errata correct a printed minimum/maximum mistake: the data field has a 46-byte minimum and a 1500-byte maximum. This guide uses ordinary untagged IPv4 framing and keeps historical trailers separate.
A familiar example
Imagine a laptop sending a small IPv4 message to a nearby machine. It finds the next local MAC address, adds the Ethernet header and enough zero padding, and sends the frame. The receiver extracts the IP bytes using IP Total Length. The chapters also show why a distant path can require smaller packets, why an off-link host uses a router’s MAC, and why knowing an address is not authentication.
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.
- Put the IP datagram after the Ethernet typeAn Internet Protocol version 4 (IPv4) datagram has its own header and data. Ethernet adds local-link addresses and a type field around it.→
- Pad a short frame without growing its IP packetEthernet requires at least 46 bytes in its data field. Padding fills the unused space around a short IP datagram; it does not become IP data.→
- Keep Ethernet size separate from path sizeThe standard Ethernet data field can hold an IP datagram of up to 1500 bytes. A later link on the route may have a smaller Maximum Transmission Unit (MTU).→
- Map the next local IP address to a MAC addressBefore 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.→
- Keep historical trailers behind a compatibility gateRFC 894 also records an optional historical trailer encapsulation used by cooperating systems. It changes link-level ordering and requires positive knowledge of receiver support.→
This introduction explains RFC 894 (1984). Read the original RFC ↗