GNU/Linux Desktop Survival Guide by Graham Williams
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Router to Router Connections

Suppose you have, for example, a D-Link DSL504 ADSL Moden/Router which is a 4 port router with inbuilt ADSL modem. You've bought a second (perhaps even a Belkin 54Mbps Wireless 802.11g) router 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 ADSL router as router A and the second 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 to any STATIC IP. Specify a gateway IP of 0.0.0.0 (or perhaps 192.168.111.1--some routers 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 configuration for router B should be set to STATIC with an IP address within the subnet 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 and with DHCP Disabled. In fact, router A will serve as the DHCP server for anything connected to router B.

Make sure 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.

   Router \textit{A}                   Router \textit{B}
   WAN: --> ISP modem           WAN: Empty ethernet
        Configured for ISP           Static IP with Gateway 0.0.0.0
   LAN:                         LAN:
     IP=192.168.0.1               IP=192.168.0.233
     Subnet 255.255.255.0         Subnet 255.255.255.255
     DHCP: Enabled                DHCP: Disabled
   LAN Ethernet Cabling:
     (1) <======================> (1) --> 
     (2) --> PC1                  (2) --> PC4
     (3) --> PC2                  (3) --> VoIP
     (4) --> PC3                  (4) -->

That's it! (But check out Section 66.3.5 for details on protecting your wireless connection from random access.)

Now, computers serviced by router B will be assigned DHCP by router A, within the 192.168.0.* network, together with DNS assignments. Router B is just another IP node on A's network. Any LAN computer can access and configure router B by accessing it as 192.168.0.233. All computers will be on the same network subnet and so they will have access to each other for file and printer sharing.