VLSM Variable Length subnet masks

Report
Subnetting the subnet
RD-CSY2001-2008/09
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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-CSY2001-2008/09
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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
RD-CSY2001-2008/09
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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
RD-CSY2001-2008/09
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-CSY2001-2008/09
RD-CSY2001-2008/09
RD-CSY2001-2008/09
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Address format <IP address/prefix P>.
The prefix denotes the upper P bits of the IP address.
Can be used to specify arbitrary blocks of addresses
◦ Say an ISP has 192.5.48.0, 192.5.49.0,192.5.50.0,192.5.51.0
then
◦ the IP address advertised will be 192.5.48.0/22
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An ISP can obtain a block of addresses and partition
this further to its customers
Say an ISP has 200.8.4/24 address (256 addresses)
◦ He has another customer who needs only 4 addresses from
200.8.4.24 then that block can be specified as 200.8.4.24/30
RD-CSY2001-2008/09
Net D: 7HOSTS
Net A: 14 HOSTS
Net C: 2 HOSTS
Router A
Net B: 28 HOSTS
Router B
Net E: 28 HOSTS
Given IP address 204.15.5.0/24, use VLSM to create a subnetting
scheme
RD-CSY2001-2008/09
Please try other questions on VLSM, available on module
web page
RD-CSY2001-2008/09

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