First -- you can't do your routing anywhere else; if you have an upstream router, take it out, and connect your cable modem (or other upstream connection) directly to the machine you're running the VM on.
Second -- since this virtual machine will be running NAT for your connection, your host OS will need to be getting its IP (and its connection) via a second interface on the VM.
Third -- if you have other systems on your network (and if you want wireless, you'll be taking your wireless router, disabling the "router" part of its functionality to make it a simple bridge, and using it here), you'll want to plug them into a second interface on your host (yes, this means you might need to buy a second NIC), and bridge that second interface to your guest separately. YOUR HOST SHOULD ONLY EVER HAVE AN IP ADDRESS ON THIS INTERFACE, NOT THE ONE GOING IN FROM THE CABLE MODEM.
So, from the host's perspective, what you need (if using Windows -- the way bridges work on Linux is a bit different, and IMHO saner) looks like this:
modem -> eth0 -> [VM] -> eth1 -> Router
...where...
- modem is your upstream link to the world
eth0
is a physical interface on your host WHICH HAS NO IP ADDRESS ASSIGNED, and exists only to run a bridge to the virtual machine
- Your VM has two separate virtual interfaces, bridged respectively to eth0 and eth1
eth1
is a second physical interface on your host, which your host gets its IP address and communicates to
router
is, actually, not behaving as a router at all, but as a wireless bridge. Decent routers can be reconfigured this way. If yours isn't decent, replace it.
This means you'll need to buy a second Ethernet device if you don't have one already. Don't skip that step -- attempts to do so will result in something that, if it works at all, will have only purely illusory security.