So, what to do with the old router? I've converted it to an access point, and am using it to fill a dead spot, in the furthest corner of my house. By flipping a few settings, you can turn pretty much any router into an AP, you don't even need to install any fancy third-party firmware. All you need to get started is an old router and a connection to your new router--it can be old-school wired Ethernet or you can use powerline networks to bridge the wireless divide. (We'll have more on powerline networks later this week.)
Connect to your old router The first thing to do is connect a PC to your old router. You're going to need to do a few things to make the switch from router to AP, but if you do it in the wrong order, you could end up with the router in a state that makes it difficult to connect to. Don't worry though, even if you make a mistake, the worst thing you'll need to do is perform a hard reset on the router and start over--you can't do permanent damage to the router by adjusting settings.
The easiest way to make sure you're working on the right router is to disconnect your laptop from Wi-Fi and plug it directly into the router's LAN ports. Then go to your router's IP address and log into it--typically the router's default IP address is printed on the bottom label of the router, but you can find it by going to your network control panel and browsing to the address listed as the default gateway--usually it's 192.168.0.1 or 192.168.1.1.
Match the access point settingsOnce you're logged into the router's administration settings, the first thing you'll want to do is adjust the SSID, WPA settings, encryption type, and passphrase so that they match on both your new router and the old router that you're converting into an AP (see below for an example). If the security settings don't match up, you'll have trouble moving between the two different wireless networks. The channel can be the same, or different--use a tool like InSSIDer on a laptop to find a lightly populated channel for each room.
Disable DHCP on the access pointNext, you need to disable DHCP on the AP's LAN interface. You can do this by browsing to the network or LAN section in configuration and disabling DHCP. This is really important, if you fail to disable DHCP on the AP, then you'll cause serious configuration problems on your network and all the computers connected to it will stop working properly. DHCP is what allows your router to assign local IPs to computers connected to the network. Since the new router is taking care of all the IP assignments for devices connected to it and the old router, DHCP needs to be disabled on the old router/access point.
Give your access point a private IPOnce you've disabled DHCP, you can reconfigure the LAN interface to use a private local IP address that won't conflict with the router. It's a good idea to do this so that you can easily log back into the AP later to adjust settings. To find out what the reserved range is for your new router, you should go to another computer and visit its configuration screen. Typically Linksys and D-Link routers use a range of 192.168.0.2 to 192.168.0.99 for the reserved IP addresses. It is very important that the address you choose for your AP is outside the range that your new router assigns to DHCP clients. It's worth logging into the new router to check; if you get the IP wrong, it will make your network very unreliable. You'll also need to fill in a few other settings to tell your new AP how it can connect to the Internet at large--namely DNS server, default gateway, and subnet mask. If it's required, you can use your ISP's DNS servers, or simply point the AP toward 220.127.116.11, which is Google's DNS server. Your default gateway should be the new router's IP address--typically 192.168.1.1 or 192.168.0.1. Subnet mask is typically 255.255.255.0.
Soft reboot and enjoyThat's all there is to it. Reboot your router/access point and plug it into your LAN using the router's LAN ports. (Do not use the WAN port, which is sometimes labeled Internet.) Enjoy having Wi-Fi in an otherwise dead portion of your home!
Photo credit: Scott Beale / Laughing Squid