As described in Chapter 1, the Network Layer-specific PDU is a packet. As data comes down from the Transport Layer, the Network Layer places its Layer 3 header in front of the segment received and this group becomes a packet (or a datagram). The Layer 3 header contains very important fields, including the logical address (IP address) of both the source device and the destination device.
As the frame is passed down to the Physical Layer, the information is converted into 0 and 1 bits that are sent over physical media using, for example, electrical signals (for copper links . . .
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