Just a thought, on DHCP IP addresses can change, a relatives Plusnet router maps the MAC address to the IP addresses so each individual machine can be given a recognisable name.
Great for keeping check on what's connected and when and easier to keep track of than an IP address.
Check out the DHCP section of the web interface settings. There you can enable DHCP and give static leases to devices (based on MAC addresses, like you specified).