GNU/Linux Desktop Survival Guide
by Graham Williams
Router to Router Connections
Suppose we have a typical ADSL modem with a four port router (e.g., a Fritz!Box 7390 (as of January 2012), an iiNet Bob2 (failed to get it working stably for 8 months from July 2011 and provider gave up trying to help and refunded it), or a 2Wire 2701HGV-W, or a D-Link DSL504 ADSL Modem/Router). We've bought (or have lying around) a second router (e.g., a Belkin 54Mbps Wireless 802.11g) and want to use this to add more computers to the network through ethernet cables to share drives, printers and the Internet connection. Let's refer to the first ADSL router as router A and the second as router B.
Optional: We probably don't need to limit the DHCP address range on router A--just as long as the address we use for router B has not been allocated to some other computer by router A, but my instructions used to include the following suggestion: 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, we might configure router A to only issue IP's in the range starting at 10.1.1.2 (or 10.0.0.1 or 192.168.0.2) and ending at 10.1.1.100 (or 10.0.0.137 or 192.168.0.33). Then we'll configure router B with 10.1.1.101 (or 10.0.0.233 or 192.168.0.40). This is all the setup that is required for router A, which otherwise has DHCP enabled and its usual WAN (ADSL) setup for your ISP.
Disable DHCP for router B (and perhaps we don't need the router's firewall or DMZ), and configure its WAN (Wide Area Network) type to be STATIC and set it to any IP (e.g., 10.2.2.2 or 192.168.111.2--it should be different to the A network), with a netmask of 255.255.255.0. Specify a gateway IP of 0.0.0.0 (or perhaps 10.2.2.1 or 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).
With the LAN (local area network) configuration for router B set to STATIC set an IP address within the subnet range of router A but outside its DHCP range. We might set the LAN IP to 10.1.1.101 (or 10.0.0.137 or 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. 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. The setup will look something like the following:
Router A Router B WAN: --> ISP modem WAN: Empty ethernet Configured for ISP Static IP with Gateway 0.0.0.0 LAN: LAN: IP=10.1.1.1 IP=10.1.1.101 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) --> PC5 (4) --> PC3 (4) --> PC6
That's it! (But check out Section 62.5.5 for details on protecting your wireless connection from random access.)
Now, computers serviced by router B (directly connected by Ethernet cable or else connected wirelessly) will be assigned DHCP by router A, within the 10.1.1.* (or 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 10.1.1.101 (or 192.168.0.40). All computers will be on the same network subnet and so they will have access to each other for file and printer sharing.
I added  another old router (replaced D-Link ADSL) as router C. It's network address was set to 10.1.1.102 (or 10.0.0.160). It is connected directly through to router A. Remember to disable DHCP for router C. This seemed to function without further setup.
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