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http://chrisstark.co/2013/02/cisco-icnd2-determine-the-appropriate-classless-addressing-scheme-using-vlsm-and-summarization-to-satisfy-addressing-requirements-in-a-lanwan-environment/
Cisco ICND2 - Determine the appropriate classless addressing scheme using VLSM and summarization to satisfy addressing requirements in a LAN/WAN environment