There are four physical NFC tag types. These are based on existing RFID standards; three of the tag types are based on the ISO-14443A standard, and the fourth is based on the ISO-18092 standard. This makes NFC tags at least partially compatible with many existing Mifare and FeliCa RFID systems. Although these older systems do not support NDEF, they can still identify those NFC Tags that are compatible with them. An RFID reader that can read Mifare Ultralight tags, for example, would still be able to read the UID of an NFC type 2 tag, even though it couldn’t read any NDEF data encoded on the tag.