When a computer is to be located more than 300 feet from the nearest Ethernet access point, a device known as a repeater will need to be used to maintain a signal that is strong enough to enable network communications. While you can buy a repeater to ensure that the signal strength is kept at an adequate level, you may already have an inexpensive router in your collection of extra network parts that can also serve as a repeater.
Connect your computer to the router using a standard Ethernet cable. Be sure to plug it into one of the downstream ports, not the upstream, or WAN, port.
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Open your Web browser and enter the IP address of your router. Normally, the IP address of the router will be something similar to 192.168.0.1; you should find the default address in your router's manual.
Enter the username and password. Many routers will use "admin" as the default entry for both the username and password, though you may need to consult the manual for your specific router. After the password is entered, the router will display its initial status page, with links to the various administration settings.
Locate the page that allows you to change the IP address of the router. The address should be changed so that it is not the same as the main router on the network, or any other network devices. If the main router on the network is set to use 192.168.1.1, the router that will be used as a repeater should use a higher number, such as 192.168.1.2 or 192.168.1.10, depending on how many devices are on the network.
Disable the router's DHCP server. This setting should be on the same page as the IP address, but each router is a little different. When the router is operating normally, this server assigns an IP address to every device that is connected to it. Because this router will be used as a repeater only, the IP addresses will now be handled by the main router on the network.
Turn off the DNS server, if it is activated. This function will be taken over by the main router, or by the ISP that provides the Internet connection to your location.
Disable any firewall that is built into the router. Usually, this can be done in the "Security" section, and it may require you to alter several settings, depending on the make and model of the router.
Remove any entries that are located in the "Port Forwarding" section. On some routers, this may be referred to as "Applications and Gaming," and it allows certain programs to have less-restricted access to the Internet.
Set the operating mode switch to "Gateway" or "Switch" if such a setting is available. In addition, if the router is a wireless router and has the appropriate settings, disable the wireless access or set it to function as an access point only.
Save the settings as necessary on each page, and reboot the router. After the router boots back up, the changes should allow it to act more or less as a simple repeater. Now, you simply move the router to its new location in the middle of the long cable run, connect all the cords and power it up.