If the IP 192.168.1.100 has been used in computers, the computer must be in LAN, the router's IP address is 192.168.1.1. The computer connects to the router, and the router automatically assigns a IP address to the computer. The automatic IP may be 192.168.1.100 or 192.168.1.2.
Router's default IP address is 192.168.1.1, you can change it to 192.168.1.100. After you access the router, you can change the default IP in management panel.
If you use a wireless router, you use a network cable to connect to the router and connect to each other through a wireless connection. The two computers are in a LAN segment, so that the two IP addresses of 192.168.1.100 and 192.168.1.101 are the same.