First, You are the master of Your LAN.  You can do as You please on You LAN, and there are a
number of ways to go about this.  You are very likely using a private subnet (i.e. 10.x.x.x, 172.16-31.x.x, or 192.168.x.x). You router will likely allow You to set a defined/static IP Address to be handed Your NAS based on it’s MAC address.  
Its what I do.  I setup things like My NAS, Printer, and VOIP to have a stable IP Address.
Hope this helps