Transcript Subnets

CCENT Test Strategies
Accurate and Fast IP Address Problem Solving
Part 2: Critical Reading Skills
1 April 2016
RE Meyers, Ms.Ed., CCAI
Subnets and VLSM Problem Solving
This is not a presentation on how to subnet or how
to develop VLSM IP addresses.
Both skills are presumed to be taught, practiced, and
accomplished by now.
If you still can’t subnet or develop VLSM addresses, then
you need to go back and practice those skills.
Practice is the only way to become proficient at
subnetting and VLSM addressing.
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Very few people actually like to practice.
Nobody likes to fail.
Practice reduces failure to a small percentage.
Your choice.
RE Meyers, Ms.Ed., CCAI
Subnets and VLSM Problem Solving
Given enough practice and time, networking
instructors and students will be able to solve all
subnet and VLSM problems on an exam.
On a certification exam, the biggest challenge is time.
• Certification exam time management is a critical skill for success.
• Spending the least amount of time on each question to arrive at
fast and accurate answers is vital.
• Under pressure of a ticking clock, we all can choke and freeze.
• This lesson proposes a way to solve time-consuming subnet and
VLSM problems fast and accurately.
• It still requires patience and practice.
• Did I mention practice? You must practice. A lot.
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RE Meyers, Ms.Ed., CCAI
Critical Reading
Many complain that Cisco exams are “reading
comprehension exams” and not technical exams.
Anyone who works at a help desk will confirm that clear
communication is the first hurdle to solving any problem.
If you don’t understand the question, then any technical
solution is lost.
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RE Meyers, Ms.Ed., CCAI
Critical Reading: Eliminate Distracters, Find the Keys
Word problems and logical topologies always give:
Exactly what you need, Keys, to solve the problem.
More than you need, called Distracters.
The first step to quick problem solving is eliminate
the Distracters and pick out the Keys to the
solution.
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RE Meyers, Ms.Ed., CCAI
IP Problem Solving: Eliminate Distracters, Find the Keys
Example:
 A network engineer assigned the following addresses to
serial connections on his enterprise network:
192.168.1.137/30, 192.168.1.209/30. 192.168.202/30,
and 192.168.1.242/30. What class of addressing is
being used? (select two)
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A
B
C
RFC 1918
RFC 1919
PAT
RE Meyers, Ms.Ed., CCAI
IP Problem Solving: Eliminate Distracters, Find the Keys
Red distracters. Blue keys:
 A network engineer assigned the following addresses to
serial connections on his enterprise network:
192.168.1.137/30, 192.168.1.209/30. 192.168.1.202/30,
and 192.168.1.242/30. What class of addressing is
being used? (select 2)
• Serial connections: Who cares? The question is about address
class not connections.
• 1.137/30, 1.209/30, 1.202/30, 1.242/30: The third and fourth octet
and CIDR do NOT determine the class of the address.
• 192.: The first octet number determines the IP Class.
• 192.168.1.x: The first octets of this IP will also tell you if it is a
special reserved IP class.
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RE Meyers, Ms.Ed., CCAI
IP Problem Solving: Use the Keys
Since you can’t highlight the PC screen, you are
allowed to use pen and paper on exams.
USE THEM and write down the keys from the question.
• 192. The first octet number determines the IP Class.
– Class C
• 192.168. A special reserved IP class
– Class C private address defined by RFC 1918
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RE Meyers, Ms.Ed., CCAI
IP Problem Solving: Pick the Correct Answers
 A network engineer assigned the following
addresses to serial connections on his enterprise
network: 192.168.1.137/30, 192.168.1.209/30.
192.168.1.202/30, and 192.168.1.242/30. What
class of addressing is being used? (select 2) :
A
Any address beginning with 192 is Class C.
B
C
Addresses beginning with 192.168. are private RFC 1918.
RFC 1918
RFC 1919
PAT
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RE Meyers, Ms.Ed., CCAI
1) Finding Network Addresses: Subnetting a Subnet
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RE Meyers, Ms.Ed., CCAI
1) Finding Network Addresses: Subnetting a Subnet
Eliminate Distracters, Find the Keys
Router names, port id’s (S0, E0)
192.168.1.64/26
192.168.1.128/26
“…valid VLSM network addresses for the serial link…”
1 April 2016
RE Meyers, Ms.Ed., CCAI
1) Finding Network Addresses: Use the Keys
192.168.1.64/26 and 192.168.1.128/26 use up a lot
of IP addresses.
What ranges are used and what ranges are
available?
/26 means 2 bits are borrowed, so network addresses
start at 0 and increment by 64:
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Subnet 0 = 192.168.1.0 – 63 /26 Unused; Range Available
Subnet 1 = 192.168.1.64 – 127 /26 Used; unavailable
Subnet 2 = 192.168.1.128 – 191 /26 Used; unavailable
Subnet 3 = 192.168.1.192 – 255 /26 Unused; Range Available
RE Meyers, Ms.Ed., CCAI
1) Finding Network Addresses: Use the Keys
Solve the problem
 Subnet 0 = 192.168.1.0 – 63 /26 Unused; Range Available
 Option 1: 192.168.1.4/30 increments the networks by 4 and falls inside
the subnet 0 range which is available.
 Option 2: 192.168.1.8/30 is the next network after 4/30 and also falls
inside the subnet 0 range which is available.
 Subnet 1 = 192.168.1.64 – 127 /26 Used; Range Unavailable
X Options 3 & 4 are inside this range and NOT available.
 Subnet 2 = 192.168.1.128 – 191 /26 Used; Range Unavailable
X Option 5 is inside this range and NOT available
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RE Meyers, Ms.Ed., CCAI
1) Finding Network Addresses: Pick the Correct Answers
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RE Meyers, Ms.Ed., CCAI
2) Finding a Range of Hosts, Ex. A
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RE Meyers, Ms.Ed., CCAI
2) Finding a Range of Hosts, Ex. A
Eliminate Distracters, Find the Keys:
10.118.197.55/20 assigned to Host A
“How many additional networked devices will this
subnetwork support?”
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RE Meyers, Ms.Ed., CCAI
2) Finding a Range of Hosts, Ex. A: Use the Keys
10.118.197.55/20
The IP address is not important.
 /20 tells you that 12 bits are used for
the host range.
212 – 2 = 4096 – 2 = 4094
4094 – 1 host already assigned =
4093
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RE Meyers, Ms.Ed., CCAI
2) Finding a Range of Hosts, Ex. A: Pick the Answer
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2) Finding a Range of Hosts, Ex. B
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RE Meyers, Ms.Ed., CCAI
2) Finding a Range of Hosts, Ex. B
Eliminate Distracters, Find the Keys
Router Serial connection and cloud
192.168.65.32/27 is a subnetwork because of /27
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RE Meyers, Ms.Ed., CCAI
2) Finding a Range of Hosts, Ex. B: Use the Keys
192.168.65.32/27
/27 indicates 3 bits were borrowed.
Network addresses start at 0 and increment by 32.
• Subnet 0 = 192.168.65.0 – 31 /27
• Subnet 1 = 192.168.65.32 – 63 /27
– All host addresses from 192.168.65.33 through
192.168.65.62/27are available for use.
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RE Meyers, Ms.Ed., CCAI
2) Finding a Range of Hosts, Ex. B: Pick the Answers
All host addresses from 192.168.65.33
through 192.168.65.62/27are available
for use.
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RE Meyers, Ms.Ed., CCAI
3) Find Network Information from a Host IP
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RE Meyers, Ms.Ed., CCAI
3) Find Network Information from a Host IP
Eliminate Distracters, Find Keys:
Distracters are also found in answer options!
In this case, all of the answers are very distracting, and
three are very wrong!
Concentrate on what you can learn from the given IP
• “… information…from 192.168.2.93/29”
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RE Meyers, Ms.Ed., CCAI
3) Find Network Information from a Host IP: Use the Keys
192.168.2.93/29
 /29 indicates 5 bits were borrowed
 /29 is CIDR for subnet mask 255.255.255.248
 Networks increment by 8.
 0 – 7, 8 – 15, 16 – 31, … 72 – 79, 80 – 87, 88 –
95, etc.
 Usable hosts per network are 23 -2 = 6
 .93 is the 5th host on the .88 – .95 network
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RE Meyers, Ms.Ed., CCAI
3) Find Network Information from a Host IP: NOTE!
Use your correct work to eliminate wrong options
• /29 indicates 5 bits were borrowed
• /29 is CIDR for subnet mask 255.255.255.248
• Networks increment by 8.
• 0 – 7, 8 – 15, 16 – 31…72 – 79, 80 – 87, 88 –
95, etc.
• Usable hosts per network are 23 -2 = 6
• .93 is the 5th host on the .88 – .95 network
Broadcast addresses are NEVER even
numbers!
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RE Meyers, Ms.Ed., CCAI
4) Solving a Configuration Problem, Ex. A
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RE Meyers, Ms.Ed., CCAI
4) Solving a Configuration Problem, Ex. A
Eliminate Distracters, Find Keys:
Few real distracters – ignore the
router/cloud connection.
Keys
• Router Ethernet address:
192.133.219.30/27
• Host configuration:
192.133.133.219.33 255.255.255.224
192.133.219.30
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RE Meyers, Ms.Ed., CCAI
4) Solving a Configuration Problem, Ex. A: Use Keys
Compare router port and
PC configuration:
IP of gateway matches router
port.
Router CIDR /27 means 3
bits are borrowed
• 255.255.255.224 PC and router
SM match.
CIDR /27 means networks
increment by 32
• 0 – 31, 32 – 63, 64 – 95, etc.
• IP of router port is on subnet 0.
• IP of PC is on subnet 1.
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RE Meyers, Ms.Ed., CCAI
4) Solving a Configuration Problem, Ex. A: Pick Answer
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4) Solving a Configuration Problem, Ex. B
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RE Meyers, Ms.Ed., CCAI
4) Solving a Configuration Problem, Ex. B
Eliminate Distracters, Find Keys:
H2 and H1 can’t communicate
• H1=192.168.22.30/28
• H2=192.168.22.60/28
Switch is a distracter.
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RE Meyers, Ms.Ed., CCAI
4) Solving a Configuration Problem, Ex. B: Use Keys
All you have are host IP’s and
CIDR. So one is wrong!
CIDR /28 matches on both.
• /28 borrows 4 bits, so networks
increment by 16
• 0 – 15, 16 – 32, 32 – 47, 48 – 63, etc
H1 192.168.22.30 is on subnet 1
H2 192.168.22.33 is on subnet 2
Host IP’s are not on the same
subnet.
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RE Meyers, Ms.Ed., CCAI
4) Solving a Configuration Problem, Ex. B: Pick Answer
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RE Meyers, Ms.Ed., CCAI
5) Subnetting a Subnet: Applied VLSM
A network engineer is implementing a network design
using VLSM for network 192.168.1.0/24. The engineer
has decided to take one of the subnets,
192.168.1.16/28, and subnet it further for point-to-point
serial link addresses.
What is the maximum number of subnets that can be
created from the 192.168.1.16/28 subnet for serial
connections?
1 April 2016
RE Meyers, Ms.Ed., CCAI
5) Subnetting a Subnet: Applied VLSM
Distracters and Keys
 A network engineer is implementing a network design using
VLSM for network 192.168.1.0/24. The engineer has decided
to take one of the subnets, 192.168.1.16/28, and subnet it
further for point-to-point serial link addresses.
• What is the maximum number of subnets that can be created
from the 192.168.1.16/28 subnet for serial connections?
1 April 2016
RE Meyers, Ms.Ed., CCAI
5) Subnetting a Subnet: Applied VLSM:Use Keys
192.168.1.0/24
 Complete Class C private address is used
 192.168.1.16/28
• /28 = 4 bits borrowed, subnets increment by 16
• 192.168.1.16/28 is the subnetwork address of the range
192.168.1.16 – 31/28.
Point to point serial connections require only 2 hosts
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/30 CIDR will provide 2 useable hosts per network.
Start at 192.168.1.16 and end at 192.168.1.31 with /30
.16 – .19, .20 – .23, .24 – .27, .28 – .31
Four subnets are available for serial connections in the
192.168.1.16/28 network.
RE Meyers, Ms.Ed., CCAI
5) Subnetting a Subnet: Applied VLSM: Pick Answer
A network engineer is implementing a network design
using VLSM for network 192.168.1.0/24. The engineer
has decided to take one of the subnets,
192.168.1.16/28, and subnet it further for point-to-point
serial link addresses. What is the maximum number
of subnets that can be created from the
192.168.1.16/28 subnet for serial connections?
Four
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RE Meyers, Ms.Ed., CCAI
Practice!
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End
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RE Meyers, Ms.Ed., CCAI