The SDLC protocol will use only an 8 bit address.The SS7 protocol, which is used in point-to-point links, does not use an address field at all.The first bit in the Address field indicates if the frame is a unicast or multicast message.A zero in the MSB bit location indicates a unicast message the remaining bits indicate the destination node address.A one in the MSB bit location indicates multicast message, the remaining bits indicate the group address. A zero in this bit location indicates the continuation of the field. A one residing in the LSB bit indicates that the length of the address field will be 8 bits long.
In many cases the address field is typically just a single byte, but an Extended Address bit may be used allowing for multi-byte addresses. The length of the address field depends on the data link layer protocol used, but is normally 0, 8 or 16 bits in length.
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