What is the best solution to enable two or more completely separate internal networks, to securely share the same broadband connection?

What is the best solution to enable two or more completely separate internal networks, to securely share the same ADSL based broadband connection?

For example, often it is desirable for there to be one high specification broadband connection installed into a large building, but where the individual departments or users within that building, need to then be on completely separate internal networks. Each separate department or user therefore would need to be able to access the Internet and possibly their own system remotely, without there being a security risk posed by the computers effectively being linked ( albeit loosely ) together.

The users in this scenario would either have simple Windows peer to peer, or Windows server based, networks.

What would therefore be the most viable way forward, without purchasing a prohibitively expensive IT solution.
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itcroydonAsked:
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masnrockCommented:
Two or more separate subnets should do the trick. So at the front end, some sort of multiple-interface router to define out all of these networks that you're mentioning. So you might take the 10.x.x.x network and break it down as you need it to be broken down.
itcroydonAuthor Commented:
Any suggestions on a suitable multiple-interface router?
masnrockCommented:
Just curious, what network infrastructure do you have in place already? Might as well try to stay consistent with the same vendor, unless you're trying to switch?
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itcroydonAuthor Commented:
We will effectively work from a 'clean sheet' in this case, although we would want to use market leading equipment, possibly Cisco, Dlink, Netgear etc.
arvindSR Manager OperationCommented:
here is the best solution for your network

http://www.xincom.com/twinwan.html

masnrockCommented:
Well, because you said a large building, maybe a Cisco 800 or 18000 series router would work? Both of those have models where your incoming connection can be ADSL. You should be able to configure any of these with NAT. Assuming you can't get as many subnets are you're looking for, you can always drop in additional cards or routers later (in a cascading fashion).
lrmooreCommented:
One potential solution would be to use a Cisco switch with Private VLANs.
Here's a low-end 8 port switch that could be a possible solution:
http://www.cisco.com/en/US/products/hw/switches/ps5213/products_qanda_item09186a00801973dd.shtml

You could have a single public IP subnet, providing each tenant their own subset of IP's and their own PVLAN. The router is in a 'community' vlan so that all private VLANS can get out the router.

One a higher-end more scalable side, you can use a 4500 series chassis switch with full Layer3 capabilities so that you can give each PVLAN their own public IP subnet. Now you have both layer 2 and layer 3 separation between clients.

Not sure of your definition of prohibitively expensive...
A much less expensive solution would be to give each client their very own $50 Linksys/Dlink router, all of their routers connect to your switch. Let their routers do all the heavy work like NAT and firewall:

      s w i t c h  --> router -->DSL----> Internet
   |    |    |      |
 R1  R2   R3    R4

Each router gets it's own public IP address from you, and the clients control what ports they want to forward to their internal network.

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itcroydonAuthor Commented:
Sorry for the delay in responding to the above comments...

When you mention that "each router gets it's own IP address from you" are you referring to allocating a fixed 192.168.X.X IP address for each router and then installing an Ethernet based router in each office with the relevant DNS settings to achieve an Internet Connection ( i.e. in a similar way to configuring a static IP address on a workstation PC ), or are you suggesting having a block of external IP addresses 212.231.16.X from the ISP and allocating each Internal router a fixed external IP address ( if so,would a standard router facilitate this? )  
lrmooreCommented:
>are you suggesting having a block of external IP addresses 212.231.16.X from the ISP and allocating each Internal router a fixed external IP address ( if so,would a standard router facilitate this? )  
Yes, this option.
Yes, most any router would facilitate this along with a switch to provide connectivity to the clients.

Internet feed --public IP--> router --public IP block-->switch----->router-1
                                           ^                                    |           WAN inteface gets public IP from you
                                      Router mode                        |          LAN interface can be default private IP
                                     Not Gateway Mode             Router-n
                                      No NAT, just pass
                                      traffic from Inet feed to switch
                                                                             
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