> (In the following, read it without the footnotes first).
>
> [ I need to start with an aside about ARP. If a machine wants to send
> a packet on the local network to a given IP address, it needs to find
> out the MAC address of the destination so that it can address the
> packet correctly at the ethernet layer, which is below the IP layer.
> This is done using a protocol called ARP. All you need to know is that:
>
> - Every packet sent must be addressed with both an IP address and a
> MAC address
> - A MAC address on the local network can be found given an IP address
> and an ARP lookup
>
> End of aside. ]
>
> A switch(*) is basically a device that joins together all of its
> ports into a single physical network. In this respect, it's like a
> hub. The only difference between a switch and a hub is that a switch
> keeps a record of which MAC addresses it has seen packets from on each
> of its ports, and a hub doesn't. This allows the switch to redirect
> packets for a given ethernet device to the right port, thus saving
> bandwidth on the other ports. A hub simply broadcasts everything
> everywhere.
>
> Thus, the switch knows nothing at all about IP addresses -- it
> works at the layer underneath that, and handles things at the ethernet
> layer.
>
> Now, to anticipate your next question: Packets for the outside
> world get sent to the router because *each machine* on the network
> knows that the router is special. This is the "default gateway"
> setting in the network options on every machine. If you are using
> DHCP, then the default gateway is set as part of the DHCP negotiation.
>
> What happens is that every device(**) that has an IP address and
> uses Internet Protocol must maintain at minimum three pieces of
> information:
>
> 1) Its IP address
> 2) Its netmask
> 3) The default gateway
>
> The IP address is used for (a) putting on outgoing packets as the
> source address, so that returned packets can be sent to the right
> place, and (b) identifying packets that are intended for the device.
> (***)
>
> The netmask is used to define the range of addresses which are
> considered "local". Addresses which match the local network are sent
> directly to the destination machine.
>
> Finally, the default gateway (which is a router device) is the
> destination for *everything else*.
>
> All of this is embodied in the routing table. On a Linux box, you
> can see the routing table thus:
>
> hrm@vlad:~ $ sudo route -n
> Kernel IP routing table
> Destination Gateway Genmask Flags Metric Ref Use Iface
> 10.0.0.0 0.0.0.0 255.255.255.0 U 0 0 0 br0
> 0.0.0.0 10.0.0.1 0.0.0.0 UG 0 0 0 br0
>
> What this says is that 10.0.0.0/24 (or 10.0.0.0/255.255.255.0) is
> the local network, and that packets for that network should be sent
> out on the br0 interface. Then packets for anywhere else (0.0.0.0/0)
> are to be sent through the default gateway (G in the Flags column) of
> 10.0.0.1, which is the router.
>
> So... if vlad, the machine above, wants to send a packet to, say,
> 10.0.0.50, it will simply put it out on the local network, with a
> destination of 10.0.0.50, and the MAC address of the destination
> machine. The switch will look at the MAC address only, and send it out
> on the correct port to get to where it's going.
>
> If vlad wants to send a packet to, say, 152.78.64.20 on the
> internet, it will construct a packet with that IP address as a
> destination and then send it to the MAC address for 10.0.0.1. Again,
> the switch looks at the MAC address only, and sends it to the gateway,
> which then processes the packet, looks at the IP address of the
> destination, and makes its own decision about where to send it next
> (using a slightly larger and more complex routing table, because it's
> a router).
>
> Hope that helps clear up any questions,
> Hugo.
>
> (*) I'm talking about unmanaged switches here -- the sort that you're
> likely to have at home. Managed switches -- the kind that get used in
> big institutional networks -- are much more complicated things.
>
> (**) "Device" here is really an *interface*, which is rather more
> complicated than "machine", in that a machine could have several
> network cards, and each network card could have several
> interfaces. But don't worry about that for now.
>
> (***) Of course, in a typical fully-switched environment, this is
> redundant.
