Forgive me if this is posted to the wrong forum.  suggestions for an alternative gratefully received.

Requirement
I need a private connection to the internet in a single NAT environment

Problem
* My office is in a business centre.
* The business centre has a shared internet service BUT it is not robust enough, and is too congested, for my needs.
* I have an adsl connection into the building.  The master socket is in the centre's comms cabinet.
* There is no direct connection between my office and the centre's comms cabinet.  Instead there are three separate switches and patch-panels daisy chained off the one in the comms centre.
* I cannot get the telephone wire from the comms cabinet into my office.  I can get a wire threaded through the ceiling panels, but the local installation company are quoting crazy fees for what should be 15 mins work.  In fact the same as one year's ADSL cost.  The business centre is unwilling (currently) to let me thread the new cable for insurance reasons.  They may change their mind and they are being as decent as can be expected.

Can anyone propose a practical solution?  

Those that i have come up with (but not validated) are as follows

ADSL Modem

hook an adsl modem into the main comms cabinet and attach the other end into the switches.  Use a router to create the PPPoA connection.  This has the negative impact of suppressing the telphony that is built into the ISP supplied box (which requires that the box function as a router).  there are SIP based workarounds but they are clunky.

anyway - would this work?  not sure how my pppoa client knows where to create the connection.

Two gateways

have the ISP supplied router installed into the main comms cabinet and set it on a different subnet to the one currently in use (in fact I think this is not possible as the router is 'for dummies' and has the IP address range fixed in the 192.168.1.* subnet.  regrettably the same as that in use by the current router...  but let's assume that i can do this.  

to avoid collisions i would have kill the dhcp function and then, in my office, T-off to another router that would handle DHCP in the right range.  

this has the advantage of being a single NAT solution (and double NAT will stop a whole bunch of things working properly) but, in order to prevent collisions on the main network i would have to use some form of authenticated DHCP method to stop non-authorised pcs getting ip address leases.  

Two gateways and multiple sub-nets

I could also use the router in my office to create a separate subnet as opposed to some funky DHCP solution.  however this would create a double nat issue.  and, for example, mobile me, still gives up with double NAT even if I put the internal router in the DMZ of the gateway.  



I'm scratching my head here, trying to save hundreds of euros in installation costs.  I may well be overthinking the issue (I have also considered powerline solutions but they are also expensive and whilst I have loads of routers and switches, I do not have any spare powerline appliances).  

Your help is greatly appreciated.
thanks
Justin

I think your "two gateways" solution is along the right track, but...
 - I'm 99% certain the ISP's router should be able to have its "inside" interface reconfigured to something other than the default.
 - In any case, you should get them to create a new VLAN on the shared network, dedicated for your office, and plug all your stuff into that and nobody else. You then have an entirely logically seperate network, regardless of whether you are using a different subnet or not. You don't need to have connection to anybody else in the centre, do you?

Thanks vince

in France (where I am) the ISP distributes a proprietary router with it's business and home connections. The routers are low functionality and locked down. Customer service is very poor.

The routers can not have their ip range changed and if you use another router you lose the (free) telephone funcionality. Annoyingly the adsl suppliers seem all to use the 192.168.1/24 subnet.

Because the centre uses a single adsl connection for all its voip and adsl connections congestion is high and qos is laughable. (15 offices in the centre). Hence the need for my own connection.