An extended Ethernet LAN is built by connecting several Ethernet segments by means of suitable devices (repeaters, bridges, etc.). A common way to make this interconnection is through a main segment, called the backbone, that joins each of the departments (or, perhaps, building floors) within the owner premises. In this kind of network, data transmission in the backbone is very sensitive to either eavesdropping or manipulation. The implementation of a cryptographic system that protects transmission (providing for both confidentiality and integrity of transmitted data) in this kind of network is addressed. The operation of the proposed system and the specific troubles encountered in implementing it in the local network of the Polytechnic University of Catalonia are presented. An analysis of the functionality of the ciphering bridges is provided and their implementation is studied in detail. Finally, the more relevant results and conclusions are given.
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