Prof Murtagh,
If a computer on an ethernet wants to send a message to a computer on a ring network (for example), the message must include the destination IP address and...a destination ethernet address? a ring network address? no second address? Does the source computer or a router provide the destination network address if the destination is on a different network? If the source computer must provide a network address for the destination, but the destination is on a different network, how does the source know what address to give?

Let's call the machine on the ethernet E, the machine on the ring network R and the router that connects E's ethernet to the rest of the internet G (for gateway, a term sometimes used for routers). Every message E sends anywhere must be sent in an ethernet packet since that is the only kind of network E has a direct connection to. So, when E sends an IP packet to B, the packet must be placed in an ethernet packet and all ethernet packets must have some ethernet address specified for their destination. E is not responsible for figuring out the "destination network address". Its only responsibility is to get the packet to a router (G). This is relatively easy since G is attached to the same ethernet as E.

Tom

tom,
do you mean "r" instead of "b" in the 3rd sentence of your response?

-grant and matt

tom,
do you mean "r" instead of "b" in the 3rd sentence of your response?

-grant and matt