When using other Data Link Layer protocols below DDP, a fundamental problem exists of address matching. This problem arises because of the two different types of addresses (Link Layer and Network Layer) which must be resolved. The matching the Link Layer address to the Network Layer address is resolved by using the AppleTalk Address Resolution Protocol (AARP), which is similar to the Address Resolution Protocol (ARP) used in the TCP/IP protocol stack. AARP is used to resolve (or bind) Ethernet addresses to DDP. When Ethernet sends frames containing DDP, the DDP packet is encapsulated by the Ethernet frame. These services, together with the mechanisms used by the Ethernet data link, are referred to as ELAP.