It is normal that you get a different IP adress when the machines reboots, it's because you are in DHCP (auto).
If you want to affect static IPs to your computer, you can do it in two ways :
1. Go to your router configuration page (look for your gateway address using "ipconfig" in windows cmd, and type this address in your browser.
From there, find your IP configuration page, and affect different static IP to your 5 computers using their MAC address of their network card (that you can find also from the "ipconfig" command). Usually, it will be addresses in 192.168.x.x /24, just pick 5 of them like 192.168.x.1, 192.168.x.2, .3, .4 and .5
By doing this, each time your computers connect to the gateway they will get the same IP address that you specified, it won't change all the time.
2. Configure each of your computers manually by going in the configuration of the network cards. There, set a static IP address (same as above) and the gateway address (which is the address of your router). For example :
Computer 1:
IP : 192.168.0.1
Gateway : 192.168.0.10 (your router)
Computer 2:
IP : 192.168.0.3
Gateway : 192.168.0.10
etc.
When you affect the IP addresses, just be careful that you don't pick one that is already used, such as the IP address of the gateway (your router).
That should work for you to fix your problem since your computers will always have the same IP address.
Regards