I know MBS said this was an over-simplification, but I think it's actually "wrong" on a couple of important points.
On Mon, Apr 12, 2010 at 11:35 AM, Michael B. Smith <[email protected]> wrote: > The way IP determines whether something is LOCAL is by issuing > an "address resolution protocol request". A computer determines whether something is local by looking at its own routing table. For most computers, the routing table is influenced by two things: The default gateway, and the computer's own IP address and netmask. The IP address/netmask defines the local network. If a packet's destination address would be on the local network, then the computer broadcasts an ARP request. Everything else gets sent to the default gateway. (The default gateway has an IP address of its own, on the local network, and that is resolved via ARP, so the computer can reach the default gateway.) > If an upstream or downstream switch has that IP connected to > their matrix, they'll respond with a proxy ARP response saying > that they'll forward it. Switches (layer two devices) do not generate or "look at" ARP requests at all. They simply forward network frames. Routers (layer three devices) respond to ARP requests. Normally, a router will only respond to an ARP request if the router's interface is configured for that IP address. Routers can also respond to ARP requests on behalf of other networks, *if* the router is configured for proxy ARP, but that's something of a rare case these days. > If there is no response to the ARP, then the message goes out the default > gateway. Once ARP gets involved, the gateway-vs-local decision has already been made. If there is no response to an ARP request, the computer will return a "Destination unreachable" error to the transmitting program. -- Ben ~ Finally, powerful endpoint security that ISN'T a resource hog! ~ ~ <http://www.sunbeltsoftware.com/Business/VIPRE-Enterprise/> ~
