If there's a more appropriate board for this issue, please let me know.

I've got a little problem that's come up unexpectedly.  At one of our temporary remote locations, we have to share our T1 line with another company.  I haven't had to mix traffic like this before so I wanted to bounce some ideas off of someone else.

I'm using a Netopia 4522 for the T1 connection and a Linksys WRT54G wireless router for the LAN connections (laptops, desktops, printers).  The Netopia has just one LAN port (currently going into the WAN port of the Linksys).  The T1 connection includes a range of something like seven IP addresses.  Two of the address are reserved (one is for the Netopia itself).  The other five addresses are being served up as DHCP addresses.  There will be a wired connection between our building and possibly two of their buildings (one on either side of us).

What I'm thinking is this:
1.  Remove an address from the DHCP pool and assign it to the Linksys.  This is something I probably should have done from the start.
2.  Remove two more addresses from the DHCP pool and assign them to their routers.
3.  Install a small unmanaged switch.  Connect the Netopia to the uplink port of the unmanaged switch.  Connect my Linksys and whatever they're using to the LAN ports.  I realize that it would probably be better to use a managed switch with VLANs but I'm kinda limited in what I can do right now.

Would this sufficiently separate our traffic from a security standpoint?  Neither party should be able to fire up a packet sniffer and eavesdrop on the other's communication, correct?  The other reason for this setup is to assign external IP address to their routers so that their IT people can get into their network if needed.

I'm already dreading what this is going to do for our throughput.  It's bad enough with two full-time and three part-time users.  This other company may add another eight or so.

Any ideas or suggestions?

I was involved in implementing a similar situation. Used an unmanaged switch to separate the paths  One path used Cisco? PIX and I seem to remember the other path's hardware as SonicWall.  It gave us parallel routing between a windows environment and a unix environment serving Atlanta and West Palm Beach.
The customer later added security cameras using a 3rd IP address.
I would suspect that you will need something to limit usage on both branches. Perhaps a managed switch.

Ed Fair
Give the wrong symptoms, get the wrong solutions.