Multi-link device (MLD) devices and transitions are described. The MAC addresses of the AP MLD and non-AP MLD are used to generate keys for different fast transitions (FT) between a non-AP MLD and an AP MLD. In an FT initial mobility domain operation, the AP MLD MAC address is used as the R1KH-ID and the non-AP MLD MAC address is used as the S0KH-ID and S1KH-ID. The MAC addresses are exchanged in Authentication Request/Response or Association Request/Response messages and the GTK/IGTK/BIGTK are delivered in a single FT 4-way handshake. In a fast MLD transition to an AP MLD in the same ESS, the other AP MLD MAC address is used as the R1KH-ID and the non-AP MLD MAC address is used as the S1KH-ID. The MAC addresses are exchanged in Authentication Request/Response or Association Request/Response messages and the GTK/IGTK/BIGTK are delivered in an FTE.