0x01ffc9a7

0x01ffc9a7 is a four-byte hexadecimal value used in Ethereum smart contracts as the interface identifier for the ERC-165 standard. It denotes the interface ID that contracts implement to declare support for other interfaces via the supportsInterface function.

ERC-165 defines a single function, supportsInterface(bytes4), which allows a contract to advertise which interfaces it implements. The interface ID 0x01ffc9a7 is derived from the function selectors of the interface. Since ERC-165 contains only supportsInterface, the interface ID equals the selector of that function, which is 0x01ffc9a7 when computed as bytes4(keccak256('supportsInterface(bytes4)')).

In practice, contracts that implement ERC165 expose supportsInterface and return true when queried with 0x01ffc9a7, signaling compliance with ERC-165. Other standards may use ERC165 to advertise their own interface IDs, enabling clients to detect support programmatically.

Common related interface IDs include 0x80ac58cd for ERC-721 and 0xd9b67a26 for ERC-1155, both using ERC-165 for interface detection. It is important to note that supportsInterface only indicates support for an interface ID and does not guarantee full compatibility or behavior across all functions of that interface. Some projects may implement ERC165 partially or not at all.

Related concepts include ERC-165 and other interface standards such as ERC-721 and ERC-1155.