Don't throw it away!
You have just bought a new one router and you don't know what to do with the old (working) one? Maybe you put it in a box, or a drawer, or you forgot it in a closet. However and wherever it is, it can be reused.
(and if you still need to change your router, here's where you can buy it)
1) Use the router as an inexpensive network switch
Most routers have no more than six Ethernet ports. Indeed, this figure very often stops at only four. It is clear that, with today's need to connect so many devices via Ethernet, the ports end immediately.
For example, smart TVs, set-top boxes, game consoles, home appliances control tools and more, may lack wireless networking capabilities, need to be physically connected to the network, right through Ethernet ports.
And if the doors run out, more can be added by one network switch. It is basically the "network" version of a multiple (electrical) socket, a "power strip" in short, with several ports connected to a router port.
Your old router typically has four or more ports, so connecting it to the new one will instantly increase the number of available ports. On the old router you should disable wireless networking to avoid conflicts.
2) Make it a Wireless Repeater
If your home wireless network can't cover your entire home, what do you do? You may choose to put one powerline to power outlets, or add a second router to the system.
In this last case, you will need to connect the old router to your new wireless network using the wifi signal. This way the router can share access to the wireless network, giving you wider coverage. While there may be some latency issues, it is a quick and easy way to extend your wireless network.
The uses are different, from providing a better wifi signal to a distant part of the house, to being able to stream movies on the tablet while in the garden.
3) Guest wifi connection
If your home is frequented by friends and family who use your wireless network, why not give them their own network?
This solution is a bit like the previous one (the wireless repeater), but with one difference: the router connects to the existing password-protected network, while providing password-free access to new devices.
In practice, the guest network function of the old router is used which, by default, prevents guests from accessing other devices on the network.
If this level of security does not seem sufficient to you, check the firewall settings on the main router.
4) Turn the old router into a wireless bridge
What if your new router is wireless only? If you need to connect Ethernet devices to your home network, the answer is to use a bridge wireless.
An old router can be reconfigured as a wireless bridge. It works a bit like a wireless repeater, but instead of sharing the wifi connection, the wireless bridge provides the Ethernet ports. The old router connects to an existing wifi network - just plug the devices into the Ethernet ports.
5) A tool for streaming internet radio
Do you want to listen to your favorite internet radio stations? Some routers can be configured to reproduce them, if you are experienced enough to install OpenWrt o DD-WRT, two alternative firmware for routers based on Linux distributions.
Some other software is also needed, as well as a sound card over USB to output the audio.
Obviously this is a project intended for experts and enthusiasts.DD-WRT, one of the best known alternative router firmware
6) Build a Smart Home hub
Some routers have extra ports. In some cases, these can be USB ports, through which you can, for example, change or update the firmware of the router.
Other devices may have a serial port: these routers can be reconfigured as a server for home automation.
Basically, the router acts as a server to which you can connect with the browser. Either from a PC, or for convenience from a smartphone.
In this site explains (in English) how to create an elementary smart home configuration using Arduino, the router and some RF controlled switches.
8) Convert the router to a NAS
Are you looking for a way to store your data on a single device and access it from anywhere in your home? If so you need a NAS (Network Attached Storage), which in practice is one (or more) hard disk connected to the local network.
While NAS are quite cheap, if you have an old router you can save some more money. We are talking, however, of routers capable of running on custom firmware such as DD-WRT and which are equipped with a USB port. You should also be able to see the contents of any USB device connected through the router (without USB there is no way to connect the hard drive or USB sticks).
Once configured, your custom NAS should give you instant access to your files from anywhere in your home.
9) Build your own VPN router
Old routers that have custom firmware installed (like the ones we talked about in point 5) can be configured with VPN software. This means that if you have an account with one of the many paid VPNs out there, it can be configured on your router.
As a result, each device can be protected by the VPN, and you will no longer need special programs when connecting via your home network.
Some older routers already support setting up a VPN, but this works when configured in "modem-only" mode.
10) Make some money
If you don't feel like wasting time trying to configure your old router with modern hardware, why not sell it directly? Put the brand and model and probably someone will buy it for you (maybe you won't get much out of it but it's better than keeping it collecting dust in some drawer ..).
Do you still have to change the router? Buy it here