I was thinking of sending this as a hint but I think it's either something well known or not of much interest.

In MacOSX it's very easy to use the same ethernet card for two separate networks. You can do it the hard way (using the terminal and ifconfig and manually routing around) or using the network preferences control panel, duplicating your network ethernet connection and putting different parameters in it. You can have as many as 255 different networks through the same ethernet cable and ethernet card. They all have different names so they are treated as separate by the firewall programs. I am currently running three different ethernet networks in my mac, plus the modem. I am routing between them all for five users who work for me but everyone else in the networks has to use the "official" routers for their respective areas. I am also doing a bit of routing, nat and firewalling through one of the ethernet networks and through the modem link, to skip on some of the networks' restrictions.

Eduo

I know it might seem like being picky, but you have NOT created two Ethernet networks. You have created two IP networks. The two are very different things.

You are right. You can easily create two IP networks by typing in two IP addresses in the Network preferences. Even easier, you can turn on the setting, "Share the Connection with other computers on Built-in Ethernet" in the Sharing preferences. I used to do it that way myself.

The result will be that your mac will have one Ethernet card with two IP addresses. You will have two separate IP networks on the one Ethernet network. That's all ok.

You still have only one Ethernet network.

The dangers are with the lower level protocols that have no concept of IP address. A DHCP query is one of these. It can go to any machine on the Ethernet network regardless of IP address. Remember that a DHCP query is a query asking FOR an IP address. The machine is asking for an IP address because it does not yet have one.

With a cable modem, it is as if your neighbors are on your same Ethernet network. Their computers see every packet that your computer sends and vice versa, regardless of IP address. Note the random flickering on your cable modem. Those are their packets going your way.

Therefore, if either your Mac, your Airport base station, or some other device on your Ethernet network happen to have their DHCP server running, then that DHCP server can set your neighbor's IP addresses to the wrong number. That's a bad thing.

The solution is to have two Ethernet cards, creating two Ethernet networks, one for the public Internet, one for the private internal network.

For me, the question is: for all devices on my Ethernet network, do I know for SURE that there are no DHCP servers running? Do I even know for sure whether DHCP the only thing that I need to worry about? Well, I don't know, so I find it best to have two Ethernet cards. Why worry?

Besides, perfectly good used Ethernet cards cost $10 on eBay.

This kind of problem is not supposed to happen for DSL, but I also don't know for sure. Could it depend upon the service provider's implementation? Why worry?