For router to router connections Suppose you have an ADSL modem with a four port router (e.g., a D-Link DSL504 ADSL Modem/Router). You've bought a second router (e.g., a Belkin 54Mbps Wireless 802.11g) and want to plug this into the network to add in and to share more local machines, and to share the Internet connection. Let's refer to the first ADSL router as router A and the second as router B.
Configure router A to issue DHCP addresses in some range that does not include one IP address that we will use for router B For example, router A might only issue IP's in the range starting at 192.168.0.2 and ending at 192.168.0.33 and we'll configure router B with 192.168.0.40. This is all the setup that is required for router A, which otherwise has DHCP enabled and its usual WAN setup for your ISP.
Disable DHCP for router B, and configure its WAN (Wide Area Network) to any STATIC IP. Specify a gateway IP of 0.0.0.0 (or perhaps 192.168.111.1, if your router will not allow 0.0.0.0). This will stop it sending traffic to its WAN (we won't be using this router's WAN connection). Further configure the WAN Type to be Static with a WAN IP of 192.168.111.2 perhaps (should be different to the A network), and a Subnet Mask of 255.255.255.0.
The LAN (local area network) configuration for router B should be set to STATIC with an IP address within the subnet range of router A but outside its DHCP range. We might set the LAN IP to 192.168.0.40 with a Subnet Mask of 255.255.255.255 (or 255.255.255.254 perhaps if that doesn't work) and with DHCP Disabled. In fact, router A will serve as the DHCP server for anything connected to router B.
Make sure that nothing is plugged into router B's WAN. Connect a LAN ethernet port of router B to a LAN ethernet port of router A to have them talking to each other, using the usual ethernet cable that you would use to plug your computer into the router.
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