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If you have the destination MAC address you need in your ARP cache, surely it shouldn't have to go through the router? Or is that the point - only the router can be sure to have all of its subnet's MAC addresses?

As a follow up: do computers on the same subnet ever regularly talk to each other without going through a router? Which protocols, if any, would do that?

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By definition, frames (what you have is a broadcast MAC address which goes on an ethernet frame, not an IP packet which uses a broadcast IP address) do not cross a layer-3 boundary. A router will strip off a layer-2 frame before forwarding the layer-3 packet encapsulated in the layer-2 frame. The router will create a new layer-2 frame with which to encapsulate the layer-3 packet for the next hop. Layer-2 broadcasts are not forwarded across a layer-3 boundary; neither are layer-3 broadcasts, except in special cases.

MAC addresses are only significant on the layer-2 domain because they are only needed for the layer-2 frame. A router will have an ARP cache for each ethernet interface which it has, just as a PC would have an ARP cache for each ethernet interface which it has. Neither a PC nor a router is sure to have all the MAC addresses for every device in the layer-2 domain.

Routers are only needed when a host needs to communicate with a host on a different layer-3 network. Any host can communicate with any other host on the same layer-2 network (there are some corner cases like Private VLANs where this may not be true) without going through a router.

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  • Okay, so let's say I'm 192.168.0.1 and I'm ARPing 192.168.0.2. Would .0.2 pick up the broadcast straight from .0.1 without any other hops? I'm really just trying to understand how layer 2 broadcast traffic works. – Hugh Mulligan Jan 21 '16 at 23:46
  • There are no other hops on the same layer-2 domain; every host is on the same LAN. Hops are layer-3 (routers). Hubs will repeat every layer-2 frame out every hub port, and switches will flood the broadcasts, multicasts, and unknown unicasts out every switch port. Every host on the layer-2 domain will see a broadcast like ARP, interrupt what it is doing, and process it to see if it needs to do something with the broadcast. – Ron Maupin♦ Jan 22 '16 at 0:01
  • Ah, so hubs and switches are responsible for every device in a LAN receiving those frames? By the sound of it, a sender will dispatch just one ff:ff:ff:ff:ff:ff frame to its hub and then the hub will relay this to all other ports. – Hugh Mulligan Jan 22 '16 at 0:55
  • Yes. Hubs are layer-1 devices which are basically the same thing as a cable, so everything connected to a hub is in the same collision and broadcast domains. Switches are layer-2 devices which divide layer-2 into collision domains, but do nothing to the layer-2 frames so they are all still on the same broadcast domain. Every host in the same layer-2 broadcast domain will receive all broadcasts in the broadcast domain. Routers are layer-3 devices which break up layer-2 broadcast domains since they will not forward broadcasts. – Ron Maupin♦ Jan 22 '16 at 0:59
  • Gotcha. And I assume that using a wireless connection changes nothing because it's still a layer-1 medium. Likewise, two devices can directly talk to each other without a hub/switch when not broadcasting, just as they could if they had an Ethernet cable connecting them. Thanks for your help. You've made this so much clearer and I know where to turn my attention to next. – Hugh Mulligan Jan 22 '16 at 1:14
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Routers interconnect layer-3 networks (i.e. IPv4, IPv6, ...) while MAC and ARP (specifically, ethernet) are layer-2 concepts. A router is not necessary for ethernet to function, or machines within the same layer-2 network to talk to each other.

Yes, computers within the same subnet talk to each other all the time.

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