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OSPF Network Design Solutions, Second Edition by Thomas M. Thomas

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Explaining the Need for VLSM and CIDR

VLSM is defined as the capability to specify a different subnet mask for the same network number on different subnets. VLSM can help optimize available address space.

CIDR is a technique that is supported by BGP-4 and based on route aggregation. CIDR enables routers to group routes to reduce the quantity of routing information that is carried by the core routers. With CIDR, several IP networks appear to networks outside the group as a single, larger entity.

Why are VLSM and CIDR needed? This answer is this: IP address depletion. This means that the current IP address scheme, which is known as IPv4, is beginning to run out of IP addresses. This is an unacceptable situation that many network engineers deal ...

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