Transcript VLSM Variable Length subnet masks

Subnetting the subnet
RD-CSY3021-2008/09
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Company with 200 hosts
Assigned class C network 195.10.1.0/24
Want 6 different LANs
Subnet class C network using mask 255.255.255.224
(/27)
Whole network has the same subnet mask or supports equal
number of hosts on all the sub-networks
◦ Is this a problem?
◦ Why?
◦ What should be done to solve it ?
RD-CSY3021-2008/09
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VLSM Subnets a subnet
◦ VLSM divides a network into subnets of various sizes to
prevent wasting IP addresses
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Uses up wasted addresses
Conserves address space on WAN links
 Point-to-point link only needs 2 addresses
 With VLSM point-to-point links can use a /30 mask
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Variable Length Subnet Masking allows division of address
space based on the size of networks
◦ Start with network requiring the most addresses
◦ Create a subnet mask
 (use CIDR – Classless Inter Domain Routing)
◦ Subnet the subnet as needed to provide address space
required for other subnets
 Be logical – start at beginning or end or address space
 Addresses must be contiguous to enable route
summarization
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Question
 If an ISP has a 200.1.1.0 Network, he wants to give
assign to 5 customers who need 20, 12, 6, 25, 18 hosts.
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What should the subnet mask be?
What is the address range in each subnet?
What should the routing table entries be?
Can it be done efficiently to conserve IP addresses?
RD-CSY3021-2008/09
RD-CSY3021-2008/09
RD-CSY3021-2008/09
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You are required to connect two networks in Brisbane
to those in Perth, having the following requirements:
Brisbane needs to support 25 hosts each on the two
networks
Perth office needs to support 5 and 10 hosts in its two
offices
You may use only 128.186.1.0 /24
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Office
#hosts
#host bits
#usable
hosts
subnet
Brisbane
25
5
30
/27
Brisbane
25
5
30
/27
Perth
10
4
14
/28
Perth
5
3
6
/29
WAN
2
2
2
/30
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Sort the subnet requirements by size, largest subnet first.
For each required subnet, allocate address space out of the
smallest available subnet
128.186.1.0/27
128.186.1.32/27
128.186.1.64/27
128.186.1.96/27
128.186.1.128/27
128.186.1.160/27
128.186.1.192/27
128.186.1.224/27
RD-CSY3021-2008/09
subnets
Hosts available
networks
1.0/27
32
1/8
Brisbane
1.32/27
32
1/8
Brisbane
1.64/28
16
1/16
Perth
1.80/29
8
1/32
Perth
1.88/30
4
1/64
WAN
1.92/30
4
1/64
1.96/27
32
1/8
1.128/25
128
1/2
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Brisbane
Perth
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Please attempt the task on VLSM posted on module web
page
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