ARP
Many network devices maintain tables of the MAC and IP addresses of other devices on the network.
On a TCP/IP network these tables are called ARP tables. They are stored in RAM.
Many devices allow the network administrators to modify the ARP table (aka ARP cache), but this is not a typical activity.
Normally, devices automatically obtain and update their own ARP tables.
Remember, in order to send data on a TCP/IP network both the MAC address and IP address are necessary.