Techniques for Solving Subnetting Questions Common questions types looked at Complicated questions section added ADR@20071.

Techniques for Solving Subnetting Questions

Common questions types looked atComplicated questions section added

ADR@2007 1

Basic Types of Questions

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First let us define the Basics

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 classe.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

• ANDinge.g. 192.168.100.5 AND 255.255.255.224 = 192.168.100.0

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More complicated question types

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Now let’s explore methods for dealing

with trickier subnetting questions

Methods for Tackling Subnetting Questions• Formula (subnet or host) Method

• Calculate a Subnet Masko Given number of subnets needed

oro Given number of hosts needed

• Calculate Number of Addresses per Subneto Given an address and subnet mask

• AND Method• Calculate a Subnet Address

o Given an IP address and masko AND/Formula Method

• Calculate Type of Addresso 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 too calculate a subnet mask

• Giveno number of subnets needed

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Subnet Formula Method

• 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. 255.255.0.0 b. 255.255.240.0 c. 255.255.255.128 d. 255.255.254.0 e. 255.255.255.192

ANS

Asked too Calculate a Subnet Mask

Giveno The Number of Subnets Needed


Giveno The Number of Subnets Needed

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• 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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Subnet Formula Method: Steps

Host Formula Method

Calculate a Subnet Mask

• Asked too calculate a subnet mask

• Giveno number of hosts per subnet

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• 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. 255.255.255.252 b. 255.255.255.248 c. 255.255.255.240 d. 255.255.255.224 e. 255.255.255.128


Giveno Number of Hosts per Subnet


Giveno Number of Hosts per Subnet

ANSADR@2007 10

Host Formula Method

• 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 addresses2h - 2 = useable number addresses

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Host Formula Method: Steps

Host Formula Method Reversed

Calculate Number of Addresses per Subnet

• Asked too calculate the number of addresses in each subnet

(total or useable)

• Giveno address and mask

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• 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. 6 b. 14 c. 30 d. 62

Asked too Calculate Number Addresses per Subnet

Giveno Address and Mask

Asked too Calculate Number Addresses per Subnet

Giveno Address and Mask

ANSADR@2007 13

Host Formula Method Reversed

• Examine the subnet mask and determine how many host bits (h) there are.

• Use 2h formula to calculate total number addresses per subnetor

• Use 2h - 2 formula to calculate useable number of addresses per subneti.e.

number of hosts = 2h -2

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Host Formula Method Reversed: Steps

• Asked too calculate a subnet address

• Giveno an IP address and mask

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ANDING Method

Calculate Subnet Addresses

• Which address is a valid subnet address if a 26 bit mask is used for subnetting?

a. 172.16.192.252 b. 172.16.128.32 c. 172.16.43.16 d. 172.16.243.64 e. 172.16.157.96

Asked too Calculate a Subnet Address

Giveno An IP Address and Mask

Asked too Calculate a Subnet Address


ANSADR@2007 16

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 Method

• Asked too Calculate the Type of Address

• Giveno An IP Address and Mask

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ANDING/Formula Method

Calculate Address Type

• Which type of address is 192.168.110.112/28?

a. host address b. subnet address c. broadcast address d. multicast address

Asked too Calculate the Type of Address


Asked too Calculate the Type of Address


ANSADR@2007 19

ANDING/Formula Method

• AND the IP address and subnet mask to find the home subnetDoes 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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ANDING/Formula Method: Steps

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 mask2n = 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 mask2h -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 subnetUse the 2h formula to find the size of the subnet. Add this number -1 to

the resident subnet to find the broadcast addressUse the above information to decide on the type of IP address

• Pie Chart MethodUse 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 usefor the following questions

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What Question Type is This?

• 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?

Host Formula Reversed MethodAsked to

o Calculate Number Addresses per SubnetGiven

o Network Address and Mask

Host Formula Reversed MethodAsked to

o Calculate Number Addresses per SubnetGiven

o Network Address and Mask

ANS

Click to show method

a. 510 b. 254 c. 30

d. 14 e. 6f. 2

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• 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?

ANDING MethodAsked to

o Calculate the Subnet an Address Belongs ToGiven

o An IP Address and Mask

ANDING MethodAsked to

o Calculate the Subnet an Address Belongs ToGiven


ANS

a. 172.16. 30.0 b. 172.16. 30.128 c. 172.16. 30.160

d. 172.16. 30.168 e. 172.16. 30.176f. 172.16. 30.254


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• 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

Host Formula MethodAsked to

o Calculate a Subnet MaskGiven

o Number of Hosts per Subnet

Host Formula MethodAsked to


o Number of Hosts per Subnet

ANS


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• Which type of address is 192.168.110.127/28?

• host address • subnet address • broadcast address • multicast address

ANDING/Formula MethodAsked to

o Calculate the Type of AddressGiven


ANDING/Formula MethodAsked to

o Calculate the Type of AddressGiven


ANS


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• 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

Formula Method (host and subnet)Asked to


o Number of Hosts per Subnet & Number Subnets

Formula Method (host and subnet)Asked to


o Number of Hosts per Subnet & Number Subnets

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

• 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.

a. The LAN is being addressed properly. b. The router LAN interface is being assigned a subnet address. c. The router LAN interface is being assigned a broadcast address. d. The subnet mask does not allow enough host addresses in a single subnet

ADR@2007

ANS

30

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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• The addressing scheme shown above is being planned for an office. What is the status of the proposed network?

a. The configuration will work as planned. b. The subnet mask of host A is incorrect. c. The default gateway of host A is a network address. d. The addresses on the router LAN interfaces are on the same subnet. e. The IP address of host A is on a different subnet than the subnet that the

Ethernet router interface ADR@2007

ANS

32

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


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• Host A is connected to the LAN, but it cannot connect to the Internet. What are the two problems with this configuration?

a. The default gateway is a network address. b. The default gateway is on a different network than the host. c. The router serial interface is on the same network as the LAN interfaced. The host subnet mask is incorrect.

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ANS

34

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


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Complicated Example 3• Draw a diagram

click for each step

.64

1. Work out how many subnets for /27

3 bits = 8 subnets draw subnets on diagram

2. Work out how many addresses in each subnet5 bits left = 32 addresses in each Label subnet addresses diagram

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

5. Work out the full subnet mask for /27Subnet mask is 255.255.255.224

.32

.96

A

0

1

2

3

5

4

7

6

.128

.160

.192

.224

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• 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?

a. IP address: 192.168.10.78 mask: 255.255.255.240 gateway: 192.168.10.79 b. IP address: 192.168.10.78 mask: 255.255.255.240 gateway: 192.168.10.65 c. IP address: 192.168.10.79 mask: 255.255.255.248 gateway: 192.168.10.62 d. IP address: 192.168.10.254 mask: 255.255.255.0 gateway: 192.168.10.1

ADR@2007

ANS

37

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

.64 Sub addr

.80 Sub addr

.79 Br addr

1. Work out how many subnets for /284 bits = 16 subnets draw subnets on diagram

3. Work out how many addresses in each subnet4 bits left = 16 addresses in each Label fifth subnet on diagram

4. Assign first address in fifth subnet to gatewayFirst address is .65

5. Assign last address in fifth subnet to serverLast address is .78

2. Work out the full subnet mask for /28Subnet mask is 255.255.255.240

0

1

2

3

4

5

67

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End of Presentation

practice, practice, practice

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Techniques for Solving Subnetting Questions Common questions types looked at Complicated questions section added ADR@20071.

Documents

subnet mask o

subnet o

subnet mask adr

subnet adr

subnet address o

appropriate subnet mask

correct subnet mask

subnet bits