• Logical addresses are necessary for universal communications that are independent of underlying physical networks.
• Physical addresses are not adequate in an internetwork environment where different networks can have different address formats.
•  A universal addressing system is needed in which each host can be identified uniquely, regardless of the underlying physical network.
• The logical addresses are designed for this purpose. A logical address in the Internet is currently a 32-bit address that can uniquely define a host connected to the Internet.
• No two publicly addressed and visible hosts on the Internet can have the same IP address.

Example

• Figure shows a part of an internet with two routers connecting three LANs.
• Each device (computer or router) has a pair of addresses (logical and physical) for each connection.
• In this case, each computer is connected to only one link and therefore has only one pair of addresses.
• Each router, however, is connected to three networks (only two are shown in the figure).
•  So each router has three pairs of addresses, one for each connection.

Example of Logical addressing

• Although it may obvious that each router must have a separate physical address for each connection, it may not be obvious why it needs a logical address for each connection.
• Router consults its routing table and ARP to find the physical destination address of the next hop (router 2), creates a new frame, encapsulates the packet, and sends it to router 2.
• Note the physical addresses in the frame. The source physical address changes from 10 to 99. The destination physical address changes from 20 (router 1 physical address) to 33 (router 2 physical address). The logical source and destination addresses must remain the same; otherwise the packet will be lost.
• At router 2 we have a similar scenario. The physical addresses are changed, and a new frame is sent to the destination computer. When the frame reaches the destination, the packet is decapsulated. The destination logical address P matches the logical address of the computer.
• The data are decapsulated from the packet and delivered to the upper layer.
• Note that although physical addresses will change from hop to hop, logical addresses remain the same from the source to destination.

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