Kiến thức về công nghệTCP IP VLSM

© 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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Address allocation for private Internets: RFC
1918
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Network Address Translation: RFC 1631
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Route summarization: RFC 1518
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Ultimate solution: IPv6 128-bit address space
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172.16.14.32/27
172.16.14. 64/27
172.16.14.96/27
What is VLSM ?
• Subnet 172.16.14.0/24 is divided into smaller subnets (Sub-
subnet):
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Subnet with one mask at first (/27)
– Further subnet one of these subnets not used elsewhere (/30)
C
B
A
HQ
172.16.1.0/24
172
.16.
2.0/
24
HQ
172.16.0.0/16

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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Exercise Calculating subnets with
Exercise Calculating subnets with
VLSM
VLSM
60 Hosts 12 Hosts 12 Hosts
Using VLSMs, define appropriate subnets for
addressing the networks using 192.168.10.0/24.
28 Hosts
KL
Sydney
Perth
Singapore
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Exercise Calculating subnets with
VLSM
60 Hosts 12 Hosts 12 Hosts
192.168.10.0/24
KL
Sydney
Perth
Singapore
192.168.10.64/27
192.168.10.128/30
192.168.10.132/30
192.168.10.136/30
28 Hosts
192.168.10.0/26 192.168.10.96/28 192.168.10.112/28
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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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1
6
8
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5
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0

/
2
1
192.168.52.0/22
192.168.48.0/20

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2
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1
6
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0
/
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Upstream
provider
Upstream
provider
Route summarization reduces routing table size by aggregating
routes to multiple networks into one supernet
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Route summarization example