The first byte(s) of a frame transmitted using the High Level Link Control (HDLC) Protocol areused to carry an address field. This field is typically a single byte, but extension is possible allowing a number of bytes to be used. The address format is shown in the figure below.
Format of the Address Byte(s) in HDLC
The address consists of three parts:
The address field is mainly used when HDLC is used in a mode which provides reliable data transfer using numbered control frames.