|I you're letting the router assign ip addresses,the printer gets it from the router.|
If you reserve a given ip address for the printer (you need to know the mac address for wifi or ethernet adapter in the printer in order for this to work) then the router will reserve that ip address for the mac address associated with it.
Equally you can set the printer to use a specific ip address too; but atill wise (necessary) to reserve it as well as above.
If you don't reserve and/o set the printer to use a specif ip address, then there is agood chance the printer will be assigned a diddrerent one at some stage. Whilst not always a problem, if you have to manually add a printer to your LAN (wifi and/or cat-5) then an ip address not, fixed as above, could become a problem when printing over the network.