Quote:

My only security issue would be if another machine out there on the internet had the same IP & mac?



That's why you use the 172 address. The 4 sets of ranges 192.etc, 172.etc, 10.etc and 169.etc as mentioned above are "unroutable". The internet "knows" about them and will not route them properly, if at all, as these ranges do not legitimately exist on the internet.

Even if you had a router setup to route any of these ranges no connections could take place as the response packets cannot route back to you. These ranges can only successfully be used behind a NAT (Network Address Translation) gateway or a proxy server.

BTW, theoretically mac addresses are globally unique. I say theoretically as I have seen both faulty NICs with damaged mac addresses and illegal clones of NICs with matching mac addresses*. However, the mac never extends past the local network. Routers, NIC's, layer 3 switches etc. all have ARP (Address Resolution Protocol) tables in their software to match IP addresses with mac addresses. If the device doesn't know the mac and the IP address is on the local network it will do a broadcast asking for the mac of the device with the required IP address, but this is getting into the protocol to a depth you don't really need...

*This casued problems that were a right royal pain to track down. About 10 years ago customer had bought cards on the cheap in Taiwan and brought them into the country "illegally". Six cards all with the same mac... When I finally figured out what was going on and fixed it (replaced the cards) the mongrel refused to pay and I couldn't even manage to recover my NIC's. Won't happen again - I'm older and wiser now...

--=-- My credo: If it's worth doing, it's worth overdoing - just ask my missus, she'll tell ya laugh --=--
You're only paranoid if you're wrong!