IP addresses on your home network are determined by your wireless router, your S8 then just uses whichever IP address that gets assigned to it. There is a setting in your phone in an advanced options menu to set a static IP but it's really important to note that this is NOT a recommended way -- if that same IP address gets reassigned to another device (another mobile device, a computer, a laptop, whatever) at some point when you're not home, when you return and your phone tries to connect itself to your WiFi there's going to be conflict. Two devices with the same IP address will create a problem. It's much better to go into the set up menu of your router and set aside an IP address for your S8, that way that IP address won't ever be used for anything else except your S8. Either turn off DHCP and manually assign any devices on your home network their own IP address, or leave DHCP enabled and give your S8 a 'reserved' IP address (probably what you want, where your S8 still gets its own IP address and any other devices will still get automatically assigned IP addresses).