|Is the DVR attached to a router in your network?|
There are a couple of ways to go.
1. In the router admin interface go into DHCP settings. As an example, DHCP may be set over a range of say 192.168.0.1 to 192.168.0.255 ( yours may not be exactly like this but it is only the last integral group that needs altering). Reduce the range to say, .0.1 to .0.100. Then in the DVR network settings manually allocate it an IP in the same subnet but outside the DHCP scope. eg 192.168.0.105
This static IP will not ever change.
Your other option may also be available within the router interface. You may alter IP lease period to never expire. That way the first IP allocated to any new device will subsequently be reserved indefinitely for that device.
Goin' Fishin' (Some day)