Four levels of addresses are used in an internet employing the TCP/IP protocols: physical (link) addresses, logical (IP) addresses, port addresses, and specific addresses

Each address is related to a specific layer in the TCPIIP architecture, as shown in fig

Physical Addresses

• The physical address, also known as the link address, is the address of a node as defined by its LAN or WAN.
• It is included in the frame used by the data link layer.
•  It is the lowest-level address.
• The physical addresses have authority over the network (LAN or WAN).
• The size and format of these addresses vary depending on the network.
• For example, Ethernet uses a 6-byte (48-bit) physical address that is imprinted on the network interface card (NIC).

Example

• In Figure a node with physical address 10 sends a frame to a node with physical address 87.
• The two nodes are connected by a link (bus topology LAN).
• At the data link layer, this frame contains physical (link) addresses in the header. These are the only addresses needed.
• The rest of the header contains other information needed at this level.
• The trailer usually contains extra bits needed for error detection.

Addressing Example:

• As the figure shows, the computer with physical address l0 is the sender, and the computer with physical address 87 is the receiver.
• The data link layer at the sender receives data from an upper layer. It encapsulates the data in a frame, adding a header and a trailer.
•  The header, among other pieces of information, carries the receiver and the sender physical (link) addresses.
• We have shown a bus topology for an isolated LAN. In a bus topology, the frame is propagated in both directions (left and right).
• The frame propagated to the left dies when it reaches the end of the cable if the cable end is terminated appropriately.
• The frame propagated to the right is sent to every station on the network.
• Each stations with physical addresses other than 87 drops the frame because the destination address in the frame does not match its own physical address.
• The intended destination computer, however, finds a match between the destination address in the frame and its own physical address. The frame is checked, the header and trailer are dropped, and the data part is decapsulated and delivered to the upper layer.
• most local-area networks use a 48-bit (6-byte) physical address
• written as 12 hexadecimal digits;
•  every byte (2 hexadecimal digits) is separated by a colon, as shown below: 07:01:02:01 :2C:4B

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