Because MS Proxy Server (MSPS) and the router both use Network Address Translation (NAT) and the router is insulated from the LAN by the NAT function in MS Proxy Server you should use Microsoft DHCP. The install wizard for Small Business Server should have configured DHCP for you.

In order to insert firewall protection between your LAN and the internet you need to install a second network interface card (NIC) in your Proxy Server computer. When you have two or more NICs in a computer it is a multi-homed device.
In order for the network to route ip packets to the correct NIC you need to install the RIP service for Internet Protocol. You can do this in the services tab in the network applet in control panel.

1. Click add and choose RIP for Internet Protocol.
2. Close the network applet.
3. You will need to reboot your computer to activate the service.

Next you need to install your second NIC. With your computer turned off, install the NIC in an empty PCI slot. When you start the computer, enter the setup utility for your CMOS. Look for plug and play configuration (not all BIOS utilities have a setting for this). Make sure it is set to other or none. If it is set to Windows 95, 98 or 2000 the system will hang (unless you are running Proxy Server on W2K). Save the configuration and exit.

After the restart, go back to the Network applet in Control panel. Follow the instructions that came with your NIC to install the drivers for it.
After you install the drivers you will be prompted to configure the card for TCP/IP. You must manually assign a TCP/IP address, netmask and default gateway. Choose a free, reserved ip address. You can view the pool of addresses in the DHCP manager under administrative tools.
10.0.0.3 through 10.0.0.9 should be available unless you have manually assigned those addresses to another device. 10.0.0.8 is a good choice and the one used in this example, but others will work also. It is absolutely crucial that the first three numbers agree with the first three numbers assigned to your router port address. That is, if you used 10.0.0.1 for set interface address on the router then the NIC to which the router is attached must use 10.0.0.x for the ip address.
Set the netmask to 255.255.255.0 and the default gateway to 10.0.0.1, again assuming you are using the numbering format in this example. Click OK, close the network applet and reboot the computer when prompted.

Small business server sets up the server and all of its services to use ip address 10.0.0.2 or 192.168.16.2. Microsoft strongly recommends you leave this address alone. If your ISP says you must configure the NIC attached to the router to use 10.0.0.2 or 192.168.16.2, they are wrong. You do not have to. DON'T RECONFIGURE THE SBS IP ADDRESS!

After removing the "File and Printer sharing" and "Client for Microsoft networks" from the ADSL network adapter, you may start getting three group policy application errors in the event viewer every 5 minutes (Q290647).

Also make sure that the Internal network adapter is configured with it's own address as the DNS and WINS server. The network adapter connecting to the ADSL modem should nothave any DNS or WINS addresses configured, and on the WINS tab for the ADSL NIC, select

"Disable NetBIOS over TCP/IP".

After the computer restarts, start the Internet Connection Wizard.