So you mean your local IP address (ie in the 10. or 192.168. subnet) or your WAN address (which identifies you on the Internet). Most likely you have DHCP enabled locally which distributes the IPs automatically when the computers connect, so if you restart it then another computer on the network may get your address. If you'd like the same address you should be able to turn on IP/MAC assignments so they get reserved ones, whilst still allowing DHCP on your router.
If you mean your IP address that identifies you on the Internet, well your ISP gives that out to you, and a new one is given whenever you reconnect to the Internet with your modem, so it would make sense that if you restart your router (assuming modem/router are the same device) then a new IP address is assigned to you.
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