When the address block in Figure 5-15 has been used up (at the time of this writing, when the 80 percent utilization level has been reached), the ISP should apply for additional address space. If good records have been kept and the documentation has been submitted to the RIR, obtaining the next block should be relatively straightforward. The RIRs generally attempt to allocate enough address space that the ISP makes a yearly request to receive resources. For example, if the ISP has consumed resources faster than documented in the rollout plan, the ISP could make a larger request the second time round and receive the larger allocation.
In the current example, it is assumed that the second allocation made is the same size as the original allocation. The RIRs quite often attempt to allocate contiguous address space—it's not guaranteed, but they usually attempt to do this. The main motivation is to encourage CIDR-ization on behalf of the ISP rather than to make life convenient.
With the second address block being contiguous, as shown in Figure 5-16, the ISP simply can extend the loopback address range for the extra equipment being installed and can keep all the addresses contiguous. The infrastructure can be expanded into the new block as shown, or the ISP can completely renumber the infrastructure into the new address block (it is relatively easy to renumber ISP infrastructure because the vast majority of it is point-to-point links between the routers and because the secondary address support makes this job very simple). Customer assignments also can commence from the top end of the block.
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