I know you are but what am I? ;-) On Mon, Apr 12, 2010 at 1:28 PM, Michael B. Smith <[email protected]> wrote: > Ttthhhhbbbbtttt :-P > > Regards, > > Michael B. Smith > Consultant and Exchange MVP > http://TheEssentialExchange.com > > > -----Original Message----- > From: Ben Scott [mailto:[email protected]] > Sent: Monday, April 12, 2010 1:27 PM > To: NT System Admin Issues > Subject: Re: Initial access to server denied, then accepted > > 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/> ~ > > ~ Finally, powerful endpoint security that ISN'T a resource hog! ~ > ~ <http://www.sunbeltsoftware.com/Business/VIPRE-Enterprise/> ~ > >
~ Finally, powerful endpoint security that ISN'T a resource hog! ~ ~ <http://www.sunbeltsoftware.com/Business/VIPRE-Enterprise/> ~
