The above discussion shows that we need another addressing mechanism in a connectionless internetwork: the link-layer addresses of the two nodes. A link-layer address is sometimes called a link address, sometimes a physical address, and sometimes a MAC address. We use these terms interchangeably in this book.
Since a link is controlled at the data-link layer, the addresses need to belong to the
data-link layer. When a datagram passes from the network layer to the data-link layer,
the datagram will be encapsulated in a frame and two data-link addresses are added to
the frame header. These two addresses are changed every time the frame moves from
one link to another. Figure 9.5 demonstrates the concept in a small internet.