Danny MacMillan wrote:
[ ... ]
I'm pretty sure I understand subnet masks.  The information I
was looking for was how my machine determines which MAC address
to put on the ethernet packet when sending to a machine off
my network.

The packet will contain the MAC address of the router.

Your machine will lookup the MAC address by doing an ARPOP_REQUEST for the IP address mentioned in the routing table which matches the destination IP address of the packet being sent (typically, using your "default" route).

First it has to know the machine is off my network,
and the network address (as determined by the IP address ANDed
with the subnet mask) is the only way I can figure that would
tell my computer that.  In fact, if I understand correctly,
that is the raison d'etre of subnet masks.  But nothing I read
about subnet masks comes out and says that directly.

Your description is right. A good primer of TCP networking ought to discuss why people use subnetting, perhaps check 'TCP/IP Network Admin' from O'Reilly.


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