In between are levels concerned with things like routing and reliability. The lowest level, the link layer, is implemented within the network adapter and its device driver. Since Ethernet is the most common type of network, we will look at it in a bit more detail. The Ethernet protocol is designed for carrying blocks of data called frames. The data portion of an Ethernet frame must be at least 38 bytes long, so filler bytes are inserted if necessary.
Frames also have a maximum size.
It is one hardware characteristic that the higher- level protocol needs to be aware of, because larger blocks of data must be fragmented into chunks that fit within the MTU and then reassembled on arrival at their destination.