Similar administrative lines are drawn between one city and another, between one state and another, and even between countries. These lines, or borders, serve the same purpose as the network portion of a Layer 3 address—that is, they allow rules to be placed on a group of end systems (in the geographic analogy, humans).
Traffic can now be specifically directed. Routing tables serve as maps and road signs.
It is very important to remember to carefully plan the placement of these boundaries to ensure the geographic proximity of the end devices or hosts. After they're defined, boundaries seldom change. This is not to say that they cannot change, however. To remain with the analogy a moment longer, remember that, historically, boundaries between cities, states, and counties are redefined. With the emergence of VLANs in recent years, this is increasingly true of networks.
NOTE In the previous analogy, access lists are the equivalent of immigration officers. Access lists are placed strategically in the hierarchical design, where one is challenged at the country level only and not at the local city level. And although flooding (excessive broadcasts) might occur at a local level (to continue the geographic analogy, think local-election brochures), these broadcasts can now be contained to ensure that buffers (mailboxes) do not overflow with unnecessary information from farther afield.
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