Networking, not

I want to use my laptop sometimes and my desktop at other times but not at the same time.

How can I set that up when the cable modem is attached to the desktop but I want to use the laptop in another room?

Surely I don’t have to buy a wireless router when I will not be using them both at the same time?

***Guess what!!! I now have a router and I am sending this from my laptop!!!***