Your network router (or other DHCP server) keeps track of how long ago it issued your computers their IP addresses. To make sure that the network does not run out of IP addresses, DHCP servers set a time limit called a lease for how long each computer can be guaranteed to keep their same address, after which the address will be re-assigned to the next device that tries to connect to it. Routers typically set a relatively short DHCP lease time limit like 24 hours but also allow administrators to change the default value. Shorter leases make sense on large networks with many devices connecting and disconnecting but generally aren't helpful on home networks. By changing your DHCP lease time to a longer value, you can increase the likelihood that each computer will keep its lease indefinitely.
Alternatively, with some more effort you can set up static IP addresses instead of using DHCP. Static addressing guarantees your computers will always use the IP address at home, but make it more difficult to move them to other networks.→ Discuss - Is an IP Address Unique to all Networks?