what's REG0 inloop0 and null0 mean when "show interface brief" on procurve

[Michaelyu]

Hi, 

I am a beginner on procurve, could anyone tell me what's the interfaces meaning of REG0, inloop0 and null0. Any document to introduce them and why the reg0 was down? Thanks in advanced.

ic1>display interface brief
Brief information on interface(s) under route mode:
Link: ADM - administratively down; Stby - standby
Protocol: (s) - spoofing
Interface Link Protocol Main IP Description
InLoop0 UP UP(s) --
M-E0/0/0 UP UP 10.80.86.70
NULL0 UP UP(s) --
REG0 DOWN -- --