A few days ago I was touting one big benefit of OS X was that you could be on multiple IP subnets simultaneously.  Today I’m writing to let you know that this still works, but beware of the dumb mac users who try this…

Today I was trying to VPN into one of my clients networks.  The PPTP VPN connection would connect, but no traffic was sent.  I glanced in the Console in OS X and found this:

3/19/09 3/19/09, 9:58:07 PM pppd[1555] route_interface: write routing socket failed, File exists

After googling “route_interface: write routing socket failed, File exists” a bit, it dawned on me, that the second Ethernet interface I added the other day, was on the same subnet as my clients remote network.  So the PPTP VPN wouldn’t pass any traffic.  Once I disabled (or I could have changed IP addresses) the second Ethernet interface, the PPTP VPN started working perfectly again!