Setting a LAN PC to a non-changing IP address is the practical way to insure it will never change. Be sure if you are using Port Forwarding or DMZ.
1) Select a unique IP for the PC
2)Obtain 1 or more (typically 2) DNS server addresses you can use
3) Most Windows PCs can use Control Panel to get to network settings (example: "Network and Dialup Connections" then "Local Area Connection" then "Properties"). Navigate to the settings for "TCP/IP" (example: "Internet Protocol (TCP/IP)").
4)Change from "Obtain an IP address automatically" to "Use the following IP address". Enter the below values:
* IP address: x.x.x.x(from step 1.)
* Subnet Mask: y.y.y.y
* Default Gateway: a.b.c.d
* DNS servers: (from step 2.)
5) Be sure and click "OK" as needed. You may be asked to reboot.