I have two interfaces connected to a real machine, eth0 and vboxnet2 (a VirtualBox bridged network).eth0 is sitting on 192.168.15.x and is connected to my ADSL modem.  vboxnet2 is on 192.168.200.x.Kernel IP routing tableDestination     Gateway         Genmask         Flags Metric Ref    Use Ifacedefault         192.168.15.1    0.0.0.0         UG    0      0        0 eth0192.168.15.0    *               255.255.255.0   U     1      0        0 eth0192.168.200.0   *               255.255.255.0   U     0      0        0 vboxnet2Should I be able to ssh from (say) 192.168.15.100 to 192.168.200.4?  My understanding of the routing table is that it says if you see any packets destined for 192.168.200.x, send them to the vboxnet2 interface.  Doesn't seem like I need NAT, iptables magic, or bridged networking for that.  But it isn't working (No route to host).Long time Linux user, but networking details have fallen off the mental stack in the last few years as NetworkManager started to do everything...
Possible to communicate between two networks without bridging?
networking
You need to enable routing with echo 1 > /proc/sys/net/ipv4/conf/all/forwarding.  Then in addition you need to set up the appropriate routes on other machines.  So on 192.168.15.100 you need to say something like ip route add 192.168.200.0/24 via 192.168.15.x (where x is whatever the forwarding machine's IP address is).  Similarly, you will need to add a route on your virtual machine for 192.168.15.0/24.