Doug wrote:


> Of course both can be used at the same time. However, normally one 
> will be the default and will handle all the traffic. If one is 
> disconnected, the other will take over.
>
> There are reports that Windows 7 can "load balance" with two 
> connections but I've not tried it since I'm running XP.
>
> However, for XP (and possibly Win 7) to use both at the same time is 
> another matter. Special software and setups may be required to fully 
> utilize them. One of the provisos is that you have two ISP's or at 
> least two primary TCPIP addresses from ISP's. 


There are several different scenarios for using multiple network
connections "at the same time".

A common one is to have connections to two different networks -- usually
a WAN/Internet connection and a local LAN connection.  In this case
which connection is used for a given packet is determined by the
packet's destination.  One of the interfaces (WAN/Internet) will be
tagged as the default, to be used for all destinations not specifically
assigned to others.
  (A "router" is a computer which, when it receives a packet not
addressed to itself, forwards it through the interface that will get it
closest to its intended destination.)

Another special case is "aggregation", where both interfaces go to the
same network backbone/infrastructure -- the effect is to create a single
connection whose capacity is roughly the sum of the individual
interfaces.  Most consumer NICs and routers/switches don't do this, but
you might run across some business-class gear that supports it. 
Aggregated links connect two devices, and so are never more than one
"hop" long.

Load balancing is a third case, where the connections are visibly
separate but advanced software distributes packets amongst the available
links -- the difference being that each link might include multiple
router hops -- with suitable equipment, expertise and coordination, a
business might connect to the Internet via multiple ISPs, possibly in
different cities.  The trickiest part isn't getting traffic to the
Internet, but getting responses back from the Internet to the same
interface as the request was sent from.  (This sort of arrangement also
can provide failover in case an ISP suffers a major equipment
failure....)

Home users rarely need to worry about more than one connection for a
computer, or occasionally two.  Larger enterprises use beefy routers and
switches to manage multiple connections, but most of that *could* be
done locally in software if the traffic volume is low enough.  

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