At least your plan has a chance most people come here and want to automatically switch between 2 ISP and then are surprised to learn how expensive and complex that is.
Not sure why you would need 50 ip. In general it is preferred to nat the ip unless you actually need a INCOMING session from the internet. This tends to only be needed for servers. The nat alone provides most the protection a firewall does. Still it will work fine as long as you can afford the IPs.
Exactly how you implement this will depend on the capability of the firewall. The simple option is to run the firewall in "transparent" mode or layer 2. Depends on the firewall what they call this feature. In this case the firewall is invisible on the network and pretty much works as a cable splice than can block selected traffic. The other way to do this will require the firewall to be able to have 2 ip addresses on a single interface or to cable 2 interface on the firewall to the same switch that runs your PC network. Again exactly how you do this will depend on the firewall.
You key to monitoring which PC is on which network is to keep track of the mac addresses of the machines. The firewall will have a list of open sessions which should show you which IP are in use and what mac addresses are associated with each ip.
Not sure what you want in a fall back system. This starts to get into the complexity of a automated failover. The simplest way to do this would be to get a junk laptop load 2 tiny virtual machines running linux and assign a ip from each network to each virtual machine. You could then leave a constant ping running in each to some internet address. You could then look at the screen and see if you are getting loss. Now of course you could load even more software to detect the loss and send email and such but that is only a tiny part of why automated internet failover gets so complex
Here's the setup:
- 2 Internet lines from different ISPs
- 1 physical firewall where the Net lines converge
- 25 users running PCs (mostly Windows 7, no Mac, no Linux)
Here's what I need to do:
- Dedicate 50 exclusive IP addresses to each modem
- Require users to manually switch their IP addresses in order to switch to the other ISP
Why I need this:
- We don't have a dedicated IT person
- I need to know who is accessing the net through which ISP at any given time (monitoring is required in case of priority uploads that demand greater bandwidth. This way I can tell non-essential users to switch over to any one ISP leaving the other line for the upload)
- A fallback system does not let me know if a network is down since the firewall automatically switches and the user remains unaffected.