Two ways..
DHCP reservation (static DHCP)
Find our the MAC address of your network card / usb wireless on your Pi.
On your router, tell it to always give a certain IP to that MAC address. Not all routers have this feature, any decent one will. What model router do you have?
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Other way is to edit your connection settings on the Pi, type in the static address that you want. Best to pick one outside the DHCP range.
Sounds complicated and Virgin super hub 2 I think