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Lab
10.7.7 Subnet Mask
Estimated time: 45 min.
Objectives:
This lab will focus on your ability to accomplish the following
tasks:
- Work with a more complex Class C subnet scenario
- Determine the subnets available with a particular IP network
address and subnet mask
- Given a network address and requirements, be able to determine how many subnets and hosts
- Be able to determine what subnet mask should be used to give the
appropriate number of hosts and subnets
- Assign IP addresses and subnet masks to hosts and router
interfaces
- Use the "ANDing" process to move an IP packet from a
local host to a remote host through a router
Background:
This lab will build on Lab Subnet Mask 1 and help develop a
better understanding of IP subnet masks using a real-world example
with additional worksheet exercises based on foundations established
in the prior lab. This
lab will focus on a Class C network with three subnets and using a
Custom Subnet Mask.
Tools / Preparation:
This is primarily a
written lab exercise but you will want to use Control Panel /
Network to review some real network IP addresses and the basics
covered in the prior lab. The
following resources will be required:
- PC workstation
with Windows operating system (Win 95, 98, NT or 2000) installed
on the PC and access to the Windows Calculator.
Notes:



Step 1 – IP Address
Basics
Explanation: For reference, the IP addressing table from the
prior lab is included here. IP network addresses are assigned by the
Internet Network Information Center (InterNIC). You will work with a
Class C.
| Cls |
1st
Octet Decimal Range |
1st
Octet High Order Bits |
Network
/ Host ID (N=Network, H=Host) |
Default
Subnet Mask |
Number
of Networks |
Hosts
per Network (usable
addresses) |
| A |
1
– 126* |
0 |
N.H.H.H |
255.0.0.0 |
126
(27 – 2) |
16,777,214
(2 24 – 2) |
| B |
128
– 191 |
1
0 |
N.N.H.H |
255.255.0.0 |
16,382
(214 - 2) |
65,534
(2 16 – 2) |
| C |
192
– 223 |
1
1 0 |
N.N.N.H |
255.255.255.0 |
2,097,150
(221 – 2) |
254
(2 8 – 2) |
| D |
224
– 239 |
1
1 1 0 |
Reserved
for Multicasting |
| E |
240
– 254 |
1
1 1 1 0 |
Experimental,
used for research |
Step 2 – Class C
network address with 3 subnets.
Task: Use the following
information and use the information from the worksheet in the prior
lab to help determine your valid subnets and host IP addresses. Do NOT
use the zero or last subnet.
Explanation: Your company has a class C network address of
200.10.57.0. You want to subdivide your physical network into 3
subnets (A, B and C) using a router as shown in the diagram at the end
of the worksheet. You will need at least 20 hosts per subnet. Answer
the following questions.
- What is the binary equivalent of the Class C network address 200.10.57.0
in the exercise?
_______
. _________ . _________ . __________
Which octet(s) represent the network portion and which octet(s)
represent the host portion of this Class C network address?

How many bits must you borrow from the host portion of the
network address in order to provide at least 3 subnets and at
least 20 hosts per subnet?

What will the Subnet Mask be (using dotted decimal notation)
based on the number of bits borrowed in step 3? _________ .
________ . __________
. _________
What is the binary equivalent of the subnet mask above:
_________ . _________ . __________ . __________
Step 3 – Class C
network address with 3 subnets.
Task:
Complete the table below according to
the instructions. Use the information in the table to answer the
questions and complete the diagram at the end of this lab.
Explanation: Be sure to specify all four octets for subnet
address and subnet mask. The same subnet mask should be used for all
hosts, router interfaces and all subnets. Having a common subnet mask
will allow hosts and routers to determine which subnet the IP packet
is intended for. Router interfaces will usually be numbered first when
assigning IP addresses and hosts will receive higher numbers.
- Fill in the following table for each of the possible subnets
that can be created by borrowing 3 bits for subnets from the
fourth octet (host octet). Identify the Network Address, the Subnet Mask, the
Subnetwork Address, the range of possible host IP addresses for each
subnet, the broadcast address of each subnet and also indicate
whether the subnet is useable or not. You will only use 3 of these
subnets for the exercise.
| SN# |
Network
Address |
Subnet
mask |
Subnetwork
Address |
Range
of possible Host IP Addresses |
Broadcast
Address |
Use? |
| 0
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- Assign an IP Address and Subnet Mask to router interface A and
write it down here.


Assign an IP Address and Subnet Mask to router interface B and
write it down here.


Assign an IP Address and Subnet Mask to router interface C and
write it down here.


Assign a host IP Address to Host X on Subnet A and assign an IP
address to Host Z on Subnet C (answers may vary). Describe the
steps (using ANDing) for the process of sending an IP packet from
Host X to host Z through the router. Remember, when ANDing, two 1s
together the result is a 1, ANDing any other combination (1 and 0,
0 and 1 or 0 and 0) results in a Zero (0). Also, when ANDing two
network IP addresses together the result of the ANDing process
will be the network (or subnetwork) address of the destination IP
address in the packet. Use the information from the diagram above
and prior lab to help assign IP addresses and subnet masks.



What is the result of the ANDing process for Host X?
Decimal Host X IP addr: _____ . _____ . _______ . ________
Binary Host X IP addr: _____ . _____ . _______ . ________
Binary Subnet Mask: _____ . _____ . _______ . ________
Binary ANDing Result: _____ . _____ . _______ . ________
Decimal ANDing Result: _____ . _____ . _______ . ________
What is the result of the ANDing process for Host Z?
Decimal Host Z IP addr: _____ . _____ . _______ . ________
Binary Host Z IP addr: _____ . _____ . _______ . ________
Binary Subnet Mask: _____ . _____ . _______ . ________
Binary ANDing Result: _____ . _____ . _______ . ________
Decimal ANDing Result: _____ . _____ . _______ . ________
The Decimal ANDing result from questions 6 is the network/subnet that Host X is on. The result from question 7 is the network/subnet that Host Z is on. Are Host X and Host Z on the same network/subnet?

What will Host X now do with the packet ?

Fill in the blanks in the following diagram with the correct
Network and IP addresses.

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