An ARP Analogy

I’ve come up with an analogy for thinking about how ARP works. I’m the kind of guy who like to hang new ideas on a framework of other ideas that I’m already familiar with. This is in that spirit. I’m not sure how sensible or useful this is going to be but let’s have a play and see where we get. I expect to find holes in the analogy at some point cos all analogies have them but it feels like it might be a pretty good model.

Think of ARP as an, admittedly slightly deranged, census surveyor (Alice) trying to find out what address Bob lives at. Here’s what all the different fields in ARP represent:

• A MAC address is a person’s name. It could represent one of the following:
  • The Ethernet Src field is Alice
  • The Ethernet Dst field is the person or people she’s talking to (this is not necessarily just Bob)
  • The ARP Target MAC is Bob (the person whose home address she’s after)
  • The ARP Src MAC is also Alice
• An IP address is a person’s physical location
  • Sender IP is Alice’s location
  • Target IP is Bob’s location
• The ARP Opcode tells us whether we’re dealing with a question about who lives where or a statement about who lives where
• An ARP Cache is a person’s address book

We’re making a distinction here between a person’s name and their home address because a person keeps their name but can move house. This is analogous to the way a NIC keeps its OSI Layer 2 MAC address but can change its Layer 3 IP address.

We’re also assuming that all this is happening on a specific street. ARP is only concerned with the local network segment, which in this analogy is the street. It doesn’t have any access or interest outside the local network segment because the rest of the network stack has already filtered all that out. This is analogous to Alice only having a list of people and addresses on the specific street she’s surveying.

So, given that setup, what actually happens? What do the various ARP messages represent?

• A broadcast request is Alice with a loudhailer asking everyone on the street where Bob lives
• A unicast request is a conversation between Alice and Bob asking him where he lives
• A broadcast response is Bob opening his window and shouting out his name and address to the whole street
• A unicast response is Bob coming outside and telling Alice his name and address

I’m going to stop there for now and have a bit more of a play with ARP in general. Then I’ll come back and fill in as many holes in all this as I can - I’m sure there are many. Watch this space…