What are the primary differences and functions (or applications) of type codes 0800 and 0806?

Through my own research, I found the 0800 was related to the 0800 to the IP/Layer 3 of the OSI model.

I also found the 0806 was related to the ARP protocol. To clarify, is ARP the protocol that makes an IP address into a MAC address in order for a switch to address a specific computer attached to a router? What layer of the OSI model is the 0806 ARP layer related to?

I am extremely new, so an in-depth explanation would be extremely appreciated.

2 Answers 2


ARP is the protocol that gives you the MAC address (layer 2) of a given IP address (layer 3).

On ethernet all communication is done using MAC addresses. Switches and other layer 2 devices only look at the MAC addresses of the packets (on layer 2 usually called frames). They don't care about the content of the frames. That is the job of layer 3.

So, for example, when your router (let's assume wants to send a packet to your PC (assume over ethernet, it has to know your PC's ethernet MAC address. It broadcasts an ARP packet to all devices on the ethernet asking for the system with address to reply. Your PC replies, and the router now knows the MAC address that it can use when sending packets to

And when your PC wants to send a packet to the router it uses the same method.

Because your PC's configuration says that is your default gateway it will also use the corresponding MAC address for all IP packets that have a destination outside your local network. So a packet for will also be sent to the router's MAC address. When the router received such a packet on its ethernet interface it is assumed to know where to forward it to based on the IP address.

  • 1
    This is an absolutely beautiful explanation and helped tremendously. Thank you.
    – beckah
    Jan 15, 2014 at 15:07

Since you are "extremely new" to networking, let me give you some pointers on the OSI model:

  1. It's just a model -- that is, a mental construct
  2. It was developed by a committee
  3. No protocols (in use) actually follow it

Many new networking students spend an inordinate amount of time trying to figure out "which layer" a protocol or function belongs to. It is a fool's errand, because whatever protocol you are looking at was not designed with the OSI model in mind. So you are forcing it to fit into an arbitrary model. That's fine, I suppose for entertainment's sake, but it won't really help you understand networking any better.

Many protocols straddle different layers of the OSI (and TCP/IP) model. Is ICMP layer 3 or layer 4? Is ARP layer 2 or layer 3? Perhaps a little of each. MPLS? Don't get me started.

The important thing about all models (here's one that things actually follow sometimes http://www.sis.pitt.edu/~icucart/networking_basics/4LayersofTCPIPModel.html ) is that there ARE layers. And layers have interfaces to the ones above and below it. So you can replace the functions of a layer with another, and as long as the interface between the other layers stays the same, everything works. So, for example, you can replace one layer 3 protocol, IP version 4, with IP version 6, and everything else will continue to work exactly as before, because IPv6 communicates with layer 2 (below) and layer 4 above) in exactly the same way as IPv4.

The other thing to remember about layers is that, for the sender, a layer encapsulates the data (technically, the protocol data unit, PDU) of the layer above it. So layer 3 encapsulates the layer 4 data unit, layer 2 encapsulates the layer 3 unit, etc. The receiver reverses the process, decapsulating the data and handing it off to the layer above it.

Sander's post about ARP, BTW is right on.

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