Fastboot command not working as expected When I connect my android phone in fastboot mode and enter fastboot devices ,I get no output at terminal ,but when I enter fastboot -i 0x1ebf devices I get an output.I need -i option with every other fastboot commands too. Why ???
The fastboot utility had an internal whitelist for USB vendor IDs, which included only some major Android device vendors:

0x0451    Texas Instruments
0x0502    Acer
0x05c6    Qualcomm
0x0955    Nvidia
0x0b05    Asus
0x0bb4    HTC
0x0fce    Sony Ericsson
0x18d1    Google
0x22b8    Motorola
0x2314    INQ Mobile
0x413c    DELL
0x8087    Intel


In the AOSP commit from 2015-08-10 this USB vendor ID whitelist was removed, therefore new fastboot versions should work with all Android devices without needing the -i 0xXXXX option (now the option just limits the selection in case multiple Android devices from different vendors are connected). However, the most current platform-tools release (23) at the time of writing this answer does not yet have this commit included, therefore you will need to keep using the -i 0xXXXX option for some time.