Let s look at Ethernet as an example of a link layer protocol. Higher level has to deal with this problem Length of data is not transmitted. When receiver notices a pause of a predetermined length, it realizes a packet has been received and it assumes the last four bytes were checksum.
Different manufacturers get ranges of addresses that they can assign to their adapters.
The IEEE standard replaces the Type field with length. Supposedly faster receives because receiver knows how many bytes are coming Notice that there is a minimum length for a packet so very small packets pay a certain overhead.