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As previously noted, the OSI protocol stack was developed mainly for large distributed systems, which typically consist of a great number of computers manufactured by different suppliers and running different operating systems. This distributed computing environment (DCE) can be located in one building, or it might be distributed among several buildings in different countries. As the number of computers in a companywide network increases, so too does the amount of administrative data.
Examples of such administrative data are the IP numbers of all computers attached to the company network and the corresponding host names. Another example would be the accounts of all employees and consultants with authorization to access the different computers. Moreover, the user accounts are organized in user groups, each of them having access rights to different resources. All of this information is maintained by NIS+, for example.
A system must fulfill several requirements if it is to handle this amount of data:
Centralized database: There must be one place that holds all of the data for the whole company.
Systemwide access to data: The data must be available across the entire network. The user's ID would have to be unique across the whole enterprise. A user ID in an international company must be known not only in one country, but all over the world.
Data reliability: The data must be correct.
Controlled access to data: Not all data should be available to everyone. For example, some private information about a user should be available only to the human resources department. At the same time, only the human resources department would have the access rights to insert new data or update existing information in its files.
The OSI protocol stack provides the DAP protocol, also known as X.500 standard or ISO 9594, to fulfill these requirements. However, the standard also has a couple of disadvantages. For one thing, to use X.500, you need to implement the whole OSI protocol stack, which is very heavy and resource consuming. As such, it is difficult to implement OSI on clients at the scale of a personal computer.
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