High-Level Data Link Control (HDLC)

[callmebefore]

The role  of all data-link  protocols is to control the correct delivery  of data over a physical link of a particular type.  For example, the Ethernet data-link  protocol uses a destination address, which is the MAC address,  field to identify the correct device that should  receive the data. 

here is the confused part which  I need your help:

 HDLC header (leased line)  has an address  field which does not clearly define either layer 2 address or layer 3 address.  so anyone out there give me what kind of address it is used by HDLC header address field.

[M02@rt37]

Hello @callmebefore 

It's worth noting that the HDLC protocol is often used as a data link layer protocol, and as such, the address field in the HDLC header is typically used to represent a Layer 2 address. However, the protocol is flexible and can be used in a variety of ways, so it is important to consider the specific implementation when interpreting the meaning of the address field.

[Richard Burts]

HDLC is used for point to point links. For protocols with multipoint capability (such as Ethernet or Frame Relay) the layer 2 link address is meaningful in identifying the appropriate destination device. But for point to point protocols you send data out the interface and it automatically arrives at the correct destination and so the link address is not meaningful and its content does not matter.

[Joseph W. Doherty]

It's a "L2" like address.  There's much to HDLC which is not used/needed in its typical p2p usage scenario.