I have approximately 15 clients on my network, I have identified all of them and modified their names for a quick and easy identification.

Occasionally, a device, an iPhone or Laptop will disappear and be replaced by a generic identification by the system.  I have to go in to that device identify it's IP address, verify that it belongs to my network and then rename it.  Why is this happening? Is it tied to the MAC address which may change depending on the iOS?  Any why to keep an id static so these changes don't occur?   Thanks.