© 2000, Cisco Systems, Inc.
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VLSM
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Issues with IP Addressing
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IP addressing crisis
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As early as 1992, the IETF identified two specific
concerns:
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IP address exhaustion
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Routing table growth
U N I V E R S I T Y
U N I V E R S I T Y
Internet
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IP Addressing Solutions
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Short term to extension to IPv4
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Subneting masking: RFCs 950, 1812
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Variable-length subnet masks: RFC 1812
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Classless interdomain routing: RFCs 1518, 1519,
2050
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172.16.14.136/30
172.16.14.132/30
172.16.14.140/30
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A waste of space
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When to use VLSM?
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Calculating subnets with VLSM
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Calculating subnets with VLSM (2)
Derived from the
172.16.32.0/20 Subnet
172.16.33.0/30
172.16.32.0/26
172.16.32.64/26
172.16.32.128/26
172.16.32.192/26
26-Bit Mask (62 Hosts)
Derived from the
172.16.33.0/26 Subnet
30-Bit Mask (2 Hosts)
172.16.33.12/30
172.16.33.8/30
172.16.33.4/30
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Route aggregation with VLSM
KL
Sydney
Perth
Singapore
192.168.48.0/24
192.168.49.0/24
192.168.50.0/24
192.168.51.0/24
192.168.52.0/24
192.168.53.0/24
192.168.54.0/24
192.168.55.0/24
192.168.56.0/24
192.168.57.0/24
192.168.63.0/24

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