Lab VLSM
40 hosts
CORE
Las Vegas
SanJose
Portland
100 hosts
100
on campus
100
Telecommute
75 hosts
Phoenix
Objective
Use VLSM to efficiently address a network.
Scenario
XYZ Company owns the Class B address 140.16.0.0. Using VLSM, the company will implement a
VLSM addressing scheme for each of its 3 domestic regional offices, and 4 international regional
offices. The company has begun to implement this strategy by supplying each of the 7 regional
offices with a VLSM subnet.
The western region of the XYZ Company has been given network 140.16.32.0/20. The San Jose
campus serves as the regional office for the western campuses. Using subnet 140.16.32.0/20, the
network administrator at the San Jose office must implement additional VLSM subnets to provide
subnets for the campus sites and the WAN links, and up to 100 telecommuters at the San Jose
campus.
Step 1
Write the VLSM subnet 140.16.32.0/20 in binary. Draw a vertical line between the network and host
parts of this address. Remember that any additional subnetting must borrow bits to the right of this
line.
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CCNP 3: VLSM v 3.0 -
Copyright  2003, Cisco Systems, Inc.
.
.
.
If no additional subnetting were performed on this network, how many hosts would this network
accommodate?
Step 2
The first round of VLSM subnetting should divide the available address space into subnets to
accommodate the existing sites, while providing for future expansion into additional campuses. If 3 of
the 12 available bits were borrowed to make VLSM subnets, how many additional subnets would be
created?
How many hosts would each of these subnets accommodate?
VLSM subnetting can be performed quite easily if the octets to be subnetted are manipulated in
binary. Since the 3rd and 4th octets of this address will be involved in subnetting, continue to
represent those octets in binary. For each round of VLSM subnetting you will follow this process:
1. Draw a line at the current subnet boundary.
2. Draw a line x binary digits to the right of the first line, where x = the number of bits borrowed in
this round.
3. Count from lowest to highest in binary “between the lines” to determine the VLSM subnets.
Step 3
Using the table below, follow the steps for this round of subnetting:
1. Draw a line between bits 20 and 21. (The current subnet boundary is /20.)
2. Draw a line 3 bits to right of the first line. (3 bits will be borrowed for this round.)
3. Beginning with 000, count in binary to find each VLSM subnet. (There will be 8 VLSM subnets
in this round.)
140
. 16
.
0
0
1
0
0 . 0
0
0
0
0
0
0
0
140
. 16
.
0
0
1
0
0 . 0
0
0
0
0
0
0
0
140
. 16
.
0
0
1
0
0 . 0
0
0
0
0
0
0
0
140
. 16
.
0
0
1
0
0 . 0
0
0
0
0
0
0
0
140
. 16
.
0
0
1
0
0 . 0
0
0
0
0
0
0
0
140
. 16
.
0
0
1
0
0 . 0
0
0
0
0
0
0
0
140
. 16
.
0
0
1
0
0 . 0
0
0
0
0
0
0
0
140
. 16
.
0
0
1
0
0 . 0
0
0
0
0
0
0
0
Campus
VLSM
Subnets
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CCNP 3: VLSM v 3.0 -
Copyright  2003, Cisco Systems, Inc.
Step 4
After the first round of VLSM subnetting, the address space has been divided into 8 subnets. Each
campus is assigned a subnet from this pool:
San Jose
Las Vegas
Portland
Phoenix
While each campus has slightly different addressing needs, it has been decided that to ensure
consistency across the network and to provide for future growth, each campus will create 4 subnets
from the network space assigned.
What prefix will these subnets have?
How many host bits do these subnets contain?
How many valid host addresses will this allow per subnet?
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CCNP 3: VLSM v 3.0 -
Copyright  2003, Cisco Systems, Inc.
Using the San Jose network as an example, complete the VLSM table to show the VLSM subnets at
the San Jose campus. Remember to follow each step to complete the VLSM subnetting for this
round. Complete the table with your values.
140
. 16
.
0
0
1
0
0
0
1
.
140
. 16
.
0
0
1
0
0
0
1
.
140
. 16
.
0
0
1
0
0
0
1
.
140
. 16
.
0
0
1
0
0
0
1
.
San Jose
Campus
VLSM
Step 5
The San Jose VLSM subnets will be used as follows:
1. Subnet 1: unassigned
2. Subnet 2: Campus file servers
3. Subnet 3: Campus hosts
4. Subnet 4: Telecommuters
What is the valid range of IP addresses (in decimal) for the campus file servers at San Jose:
Begin IP:
End IP:
Subnet Mask:
Step 6
For each of the 3 other campuses, subnet the available network space to create 4 subnets at each
campus. Complete the tables with your binary values.
Las Vegas:
140
. 16
.
0
0
1
0
.
140
. 16
.
0
0
1
0
.
140
. 16
.
0
0
1
0
.
140
. 16
.
0
0
1
0
.
Las Vegas
Campus
VLSM
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CCNP 3: VLSM v 3.0 -
Copyright  2003, Cisco Systems, Inc.
Portland:
140
. 16
.
0
0
1
0
.
140
. 16
.
0
0
1
0
.
140
. 16
.
0
0
1
0
.
140
. 16
.
0
0
1
0
.
Portland
Campus
VLSM
Phoenix:
140
. 16
.
0
0
1
0
.
140
. 16
.
0
0
1
0
.
140
. 16
.
0
0
1
0
.
140
. 16
.
0
0
1
0
.
Phoenix
Campus
VLSM
Step 7
Now that each campus has a VLSM addressing scheme, we can turn our attention to the WAN links.
As you recall, each of the campuses has one or more unassigned subnets. Since Las Vegas
currently has the smallest host requirements, one of the unassigned subnets from the Las Vegas
pool will be used to address WAN links.
Las Vegas:
140
. 16
. 0 0 1 0 0 1 0 0 . 0 0 0 0 0 0 0 0
File
Servers
140
. 16
. 0 0 1 0 0 1 0 0 . 1 0 0 0 0 0 0 0
Hosts
140
. 16
. 0 0 1 0 0 1 0 1 . 0 0 0 0 0 0 0 0
WAN
140
. 16
. 0 0 1 0 0 1 0 1 . 1 0 0 0 0 0 0 0
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CCNP 3: VLSM v 3.0 -
Copyright  2003, Cisco Systems, Inc.
If the last subnet in the Las Vegas pool is used for WAN links, it should be further subnetted with a
/30 prefix. This prefix will accommodate 2 host addresses per subnet to address the WAN links
without waste. Although this will result in too many subnets for the current WAN link needs, there will
be room for future growth.
Once again, perform each step of VLSM subnetting. Complete the table with the values of the first 4
and last 4 subnets.
140
.
16
. 0
0
1
0
0
1
0
1 . 1
140
.
16
. 0
0
1
0
0
1
0
1 . 1
140
.
16
. 0
0
1
0
0
1
0
1 . 1
140
.
16
. 0
0
1
0
0
1
0
1 . 1
**Subnets 5-28 omitted from table.**
140
.
16
. 0
0
1
0
0
1
0
1 . 1
140
.
16
. 0
0
1
0
0
1
0
1 . 1
140
.
16
. 0
0
1
0
0
1
0
1 . 1
140
.
16
. 0
0
1
0
0
1
0
1 . 1
6-6
CCNP 3: VLSM v 3.0 -
Copyright  2003, Cisco Systems, Inc.