My local environment is two LANs, one on 10.0.0/8, NAT'd to the public Internet and the other on 172.16.0/12 with all manner of Linux and embedded devices hangin' on it. There's a Windows machine that straddles the networks using 2 separate NICs. A cohort noticed the problem at the Windows box when he tried to telnet to a device of his on the 172.16 LAN and got a login prompt for an ADVA Optical Networking device somewhere in the Outernet instead. A traceroute quickly comfirmed that he wasn't going nuts. I guess it ain't up to me to tell an ISP that it's bad mojo to expose their network management nodes to their customers (downstream ISP) and *their* customers (me), but hey, this is mucking with my network! 172.16 is private and I should be able to use any part of it without fear of colliding with someone else's IPs anywhere out on The Internet.
I emailed the NOC POCs of both networks and got responses, so they'll probably work something out. In the meantime, I needed a way to corral my own two networks and keep'm separated. The Windows 'route' command seemed to fit the bill. It ain't exactly like the Linux route command, but with a little reading and testing, I came up with this:
1) Get a command promptThat seemed to do the trick.
2) > route PRINT
3) find the numbers (left column) of the interfaces on the main (outside) LAN and the development (inside) LAN
4) Restrict main LAN interface to 10/8:
> route ADD 10.0.0.0 MASK 255.0.0.0 10.0.0.1 METRIC 1 IF [main LAN interface#]
5) Restrict development LAN to 176.16/12:
> route ADD 172.16.0.0 MASK 255.240.0.0 0.0.0.0 METRIC 1 IF [development LAN interface#]
So, problem averted but not really solved. How common is it for ISPs, small or large, to route private IP space within their networks and even between their peers and downstream networks?