Linux: find MAC Address for unconfigured card

You are likely running into problems because your Zoom X5 is still acting as a router (even with those features disabled). You should look online for instructions for setting the Zoom X5 in "Bridge Mode" which will cause it to act only as a DSL Modem. This will allow your DI-524 to handle the PPPoE connection as a router and everything should work as you expect.

This link may help: http://my.ruralnetwork.net/tech/setup/dsl_setup_zoom_5554A/zoom5554a_setup11.htm

As for why the MAC address worked, often DSL providers will filter on the MAC address of their DSL modem. In your case, Verizon may have accepted also your PC MAC address after you configured it also for PPPoE. My guess is that changing the MAC address on your DI-524 had little to do with the connection working. You're probably still going to have issues because you're essentially placing a router behind a router. Your best option is to change the Zoom X5 to bridge mode and let your DI-524 be the router.

I'm using Slackware 10 as a reference, so there may be a slight variation in configuration files.

/etc/rc.d/rc.inet1 is the init script for the network interface, which is where I would begin for customizing any start time network configuration.

It looks like this init script does not have any logic for specifying the hardware address. However, a simple modification should be able to enable this functionality.

As far as your specific question, I think you're asking if specifying NIC1 as eth1 in udev will result in NIC0 defaulting to eth0? If so: probably, but I'd recommend testing this. Otherwise, please clarify.