Transcript Slide 1

Techniques for Solving
Subnetting Questions
Common questions types looked at
Complicated questions section added
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Basic Types of Questions
First let us define the Basics
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Basic Types of Questions
• These are the basics you should be comfortable with
• Knowing the default masks for the 3 network classes
e.g.
Class A == /8 == 255.0.0.0
• The default number of network and host bits for each class
e.g.
Class A == 8 network bits == 24 host bits
• Converting a CIDR mask to a subnet mask and vice-versa.
e.g.
/27 == 255.255.255.224
• ANDing
e.g.
192.168.100.5 AND 255.255.255.224 = 192.168.100.0
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More complicated question types
Now let’s explore methods for dealing
with trickier subnetting questions
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Methods for Tackling Subnetting Questions
• Formula (subnet or host) Method
• Calculate a Subnet Mask
o Given number of subnets needed
or
o Given number of hosts needed
• Calculate Number of Addresses per Subnet
o Given an address and subnet mask
• AND Method
• Calculate a Subnet Address
o Given an IP address and mask
o AND/Formula Method
• Calculate Type of Address
o Given an IP address and mask
• Pie Chart Method
• Use for any of the above types of questions (class C networks)
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Subnet Formula Method
Calculate a Subnet Mask
• Asked to
o calculate a subnet mask
• Given
o number of subnets needed
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Subnet Formula Method
Asked to
o Calculate a Subnet Mask
Given
o The Number of Subnets Needed
• A company is using a Class B IP addressing scheme and expects to
need as many as 80 networks. What is the correct subnet mask to
use with the network configuration?
a.
b.
c.
d.
e.
255.255.0.0
255.255.240.0
255.255.255.128
255.255.254.0
255.255.255.192
ANS
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Subnet Formula Method: Steps
• To calculate the subnet mask you need figure how
many subnet bits must be borrowed.
• Use the formula…
2n = number of subnets
to calculate n, which is the number of bits you need
to borrow to obtain the required number of subnets.
• Calculate the CIDR mask
• Convert the CIDR mask to full subnet mask
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Host Formula Method
Calculate a Subnet Mask
• Asked to
o calculate a subnet mask
• Given
o number of hosts per subnet
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Host Formula Method
Asked to
o Calculate a Subnet Mask
Given
o Number of Hosts per Subnet
• An IP network address has been subnetted so that every
subnetwork has 30 usable host IP addresses. What is the
appropriate subnet mask for the newly created subnetworks?
a.
b.
c.
d.
e.
255.255.255.252
255.255.255.248
255.255.255.240
255.255.255.224
255.255.255.128
ANS
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Host Formula Method: Steps
• To calculate the subnet mask, you need to figure out
how many host bits are left.
• Use the formula…
2h -2 = number of hosts
to calculate h, the number of host bits
• Calculate the CIDR mask
• Convert the CIDR mask to full subnet mask
2h = total number addresses
2h - 2 = useable number addresses
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Host Formula Method Reversed
Calculate Number of Addresses per Subnet
• Asked to
o calculate the number of addresses in each
subnet (total or useable)
• Given
o address and mask
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Host Formula Method Reversed
Asked to
o Calculate Number Addresses per Subnet
Given
o Address and Mask
• Company XYZ uses a network address of 192.168.50.0. It uses the
mask of 255.255.255.192 to create subnets. What is the maximum
number of usable hosts in each subnet?
a.
b.
c.
d.
6
14
30
62
ANS
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Host Formula Method Reversed: Steps
• Examine the subnet mask and determine how many
host bits (h) there are.
• Use 2h formula to calculate total number addresses
per subnet
or
• Use 2h - 2 formula to calculate useable number of
addresses per subnet
i.e.
number of hosts = 2h -2
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ANDING Method
Calculate Subnet Addresses
• Asked to
o calculate a subnet address
• Given
o an IP address and mask
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ANDING Method
Asked to
o Calculate a Subnet Address
Given
o An IP Address and Mask
• Which address is a valid subnet address if a 26 bit mask is used for
subnetting?
a.
b.
c.
d.
e.
172.16.192.252
172.16.128.32
172.16.43.16
172.16.243.64
172.16.157.96
ANS
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ANDING Method
•
•
•
•
Convert each IP address to binary
Convert the subnet mask to binary
AND each address against the mask
You will know if an IP address is actually a subnet
address because the answer will be the same
address.
• Hint: all subnet addresses are even numbers.
All broadcast addresses are odd numbers
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ANDING/Formula Method
Calculate Address Type
• Asked to
o Calculate the Type of Address
• Given
o An IP Address and Mask
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ANDING/Formula Method
Asked to
o Calculate the Type of Address
Given
o An IP Address and Mask
• Which type of address is 192.168.110.112/28?
a.
b.
c.
d.
host address
subnet address
broadcast address
multicast address
ANS
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ANDING/Formula Method: Steps
• AND the IP address and subnet mask to find the home subnet
Does the answer give you the same address? If yes then the IP
address is a subnet address.
• Use the formula (2h ) to work out how many addresses are in
each subnet. You need to do this so you can calculate the start
of the next subnet.
Is the IP address the last address of the subnet. If yes then the
address is a broadcast address
• If the IP address is none of the above then it is an ordinary IP
address.
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Methods Summary
• Formula (host or subnet ) Method
• To calculate a subnet mask, when you are given the number of subnets
needed
use the formula below, then work out the subnet mask
2n = number subnets
• To calculate a subnet mask when you are given the number of hosts
needed
Use the formula below then work out the subnet mask
2h -2 = number hosts
• To calculate the number of hosts per subnet, when you are given an
address and subnet mask
Use the subnet mask to work out how many bits are host bits, then
use the formula below
number hosts = 2h -2
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Methods Summary
• AND Method
• To calculate the resident subnet when given an IP address and mask
AND the IP address and mask to find the resident subnet
o AND/Formula Method Type
• To work out if an address is a subnet, broadcast or ordinary IP address
AND the IP address and mask to find the resident subnet
Use the 2h formula to find the size of the subnet. Add this number -1 to
the resident subnet to find the broadcast address
Use the above information to decide on the type of IP address
• Pie Chart Method
Use this method for any of the above types of questions for class C
networks or when a question is particularly complicated
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Determine the method to use
for the following questions
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What
Question
Type isMethod
This?
Host
Formula
Reversed
Asked to
o Calculate Number Addresses per Subnet
Given
o Network Address and Mask
Click to show method
• When a network administrator applies the subnet mask
255.255.255.252 to a Class A address, for any given subnet, how
many IP addresses are available to be assigned to devices?
a. 510
b. 254
c. 30
d. 14
e. 6
f. 2
ANS
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What ANDING
QuestionMethod
Type is This?
Asked to
o Calculate the Subnet an Address Belongs To
Given
o An IP Address and Mask
Click to show method
• A router interface has been assigned an IP address of 172.16.30.166
with a mask of 255.255.255.248. To which subnet does the IP
address belong?
a. 172.16. 30.0
b. 172.16. 30.128
c. 172.16. 30.160
d. 172.16. 30.168
e. 172.16. 30.176
f. 172.16. 30.254
ANS
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What
Question
is This?
Host
FormulaType
Method
Asked to
o Calculate a Subnet Mask
Given
o Number of Hosts per Subnet
Click to show method
• A company is planning to subnet its network for a maximum of 27
hosts. Which subnet mask would provide the needed hosts and
leave the fewest unused addresses in each subnet?
• 255.255.255.248
• 255.255.255.192
• 255.255.255.240
ANS
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What
Question Type
is This?
ANDING/Formula
Method
Asked to
o Calculate the Type of Address
Given
o An IP Address and Mask
Click to show method
• Which type of address is 192.168.110.127/28?
•
•
•
•
host address
subnet address
broadcast address
multicast address
ANS
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WhatMethod
Question(host
Typeand
is This?
Formula
subnet)
Asked to
o Calculate a Subnet Mask
Given
o Number of Hosts per Subnet & Number Subnets
Click to show
method
• A small company has a class C network license and needs to create
five usable subnets, each subnet capable of accommodating at least
18 hosts. Which of the following is the appropriate subnet mask?
• 255.255.255.192
• 255.255.255.248
• 255.255.255.224
• 255.255.224.0
ANS
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Complicated Types Questions
• These questions types usually involve a diagram and
may require combinations of the methods looked at
previously.
• The ANDING method is most often needed, although
using a pie chart can make it easier.
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Complicated Example 1
ANS
•
a.
b.
c.
d.
The addressing scheme for the LAN is 172.25.24.0/26. The hosts will be in the
first subnet. The LAN interface of the router is to be configured using
172.25.24.63 as the IP address.
The LAN is being addressed properly.
The router LAN interface is being assigned a subnet address.
The router LAN interface is being assigned a broadcast address.
The subnet mask does not allow enough host addresses in a single subnet
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Complicated Example 1
• Need to check hosts and gateway are in same subnet
• The diagram specifies the network address and mask
• Hosts addresses are given so you can work out which subnet they
are in. Need to check they are all in the same subnet and subnet
has enough room for all of them.
• Gateway address is given so you must check host and gateway are
in same subnet.
Use ANDING Method to obtain the answer or use a pie chart
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Complicated Example 2
ANS
•
The addressing scheme shown above is being planned for an office. What is the
status of the proposed network?
a.
b.
c.
d.
e.
The configuration will work as planned.
The subnet mask of host A is incorrect.
The default gateway of host A is a network address.
The addresses on the router LAN interfaces are on the same subnet.
The IP address of host A is on a different subnet than the subnet that the
Ethernet router interface
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Complicated Example 2
•
•
•
•
•
•
•
Need to check host, server and router interfaces are in appropriate subnets.
Also must check host config
How many subnets are there?
Are the host and the gateway in the same subnet
Is the host config correct.
– Is a valid IP address being used
– Is a valid mask being used
– Is a valid gateway address being used and does it match the address on
the router
Is the gateway address a valid address
Is the subnet address between the router and the server valid? Is it on a
separate subnet to the subnet with the host.
Is the server address in the correct subnet and is it a valid address
Use ANDING Method to obtain the answer or use a pie chart
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Complicated Example 3
ANS
•
a.
b.
c.
d.
Host A is connected to the LAN, but it cannot connect to the Internet. What
are the two problems with this configuration?
The default gateway is a network address.
The default gateway is on a different network than the host.
The router serial interface is on the same network as the LAN interface
The host subnet mask is incorrect.
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Complicated Example 3
• Check host and router interfaces are in appropriate subnets. Also check
host config
• How many subnets are there?
• Are the host and the gateway in the same subnet
• Is the host config correct.
– Is a valid IP address being used
– Is a valid mask being used
– Is a valid gateway address being used and does it match the address
on the router
• Is the gateway address a valid address
• Is the serial address on a different subnet or network.
• Is the server address in the correct subnet and is it a valid address
Use ANDING Method to obtain the answer or use a pie chart
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Complicated Example 3
• Draw a diagram
click for each step
.224
.32
0
7
1. Work out how many subnets for /27
3 bits = 8 subnets
6
draw subnets on diagram
1
.192
.64
2. Work out how many
addresses in each subnet
5 bits left = 32 addresses in each
A
5
Label subnet addresses diagram
2
4
3
.160
3. Which subnet is host .65 on?
Host .65 is on subnet 2
4. Which subnet is .33 gateway on?
Gateway .33 is on subnet 1
.128
5. Work out the full subnet mask for /27
Subnet mask is 255.255.255.224
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.96
Complicated Example 4
ANS
•
a.
b.
c.
d.
For network 192.168.10.0/28, an Administrator has decided he wants to use the fifth
subnet (subnet zero is the first subnet). The router interface will be assigned the first
usable address on the subnet and the server will be given the last usable address.
How should the server be configured?
IP address: 192.168.10.78 mask: 255.255.255.240 gateway: 192.168.10.79
IP address: 192.168.10.78 mask: 255.255.255.240 gateway: 192.168.10.65
IP address: 192.168.10.79 mask: 255.255.255.248 gateway: 192.168.10.62
IP address: 192.168.10.254 mask: 255.255.255.0 gateway: 192.168.10.1
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Complicated Example 4
•
•
•
•
•
•
Need to work out the subnet addresses
How many subnets are there?
How many addresses are in each subnet
Work out the address of the fifth subnet
Work out the address of the sixth subnet
Work out the host config
– IP address
– mask
– gateway address
Easiest approach here is to use a pie chart
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Complicated Example 4
• Draw a diagram
click for each step
0
1. Work out how many
subnets for /28
4 bits = 16 subnets
draw subnets on diagram
1
2
3
4
2. Work out the full subnet
mask for /28
Subnet mask is
255.255.255.240
5
7
3. Work out how many
addresses in each subnet
4 bits left = 16 addresses in each
Label fifth subnet on diagram
4. Assign first address in fifth subnet to gateway
First address is .65
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.64 Sub addr
.79 Br addr
.80 Sub addr
5. Assign last address in fifth
subnet to server
Last address is .78
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End of Presentation
practice, practice, practice
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