To determine how many IP addresses are required in your network, you should consider131 the many different locations in your network that need addresses, including headquarters, branch and regional offices, telecommuters, and so forth. The number of devices in each location must be counted, including the network devices such as routers, switches, and firewalls; workstations; IP phones; network management stations; servers; and so forth. For each of these devices, determine how many interfaces need to be addressed and whether private or public addresses will be used.
A reserve for future growth should be added to the total number of addresses required. A 10 to 20 percent reserve is typically sufficient, but the reserve should be based on your knowledge of the organization. If you do not reserve enough space for future growth, you might have to reconfigure some of your routers (for example, to add new subnets or networks into route summarization calculations); in the worst case, you might have to re-address your entire network.
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