# 8.1.4.6 Lab – Calculating IPv4 Subnets Answers

## Lab – Calculating IPv4 Subnets (Answers Version – Optional Lab)

Answers Note: Red font color or gray highlights indicate text that appears in the instructor copy only. Optional activities are designed to enhance understanding and/or to provide additional practice.

### Objectives

Part 1: Determine IPv4 Address Subnetting

Part 2: Calculate IPv4 Address Subnetting

### Background / Scenario

The ability to work with IPv4 subnets and determine network and host information based on a given IP address and subnet mask is critical to understanding how IPv4 networks operate. The first part is designed to reinforce how to compute network IP address information from a given IP address and subnet mask. When given an IP address and subnet mask, you will be able to determine other information about the subnet.

Answers Note: This activity can be done in class or assigned as homework. If the assignment is done in class, you may wish to have students work alone or in teams of 2 students each. It is suggested that the first problem is done together in class to give students guidance as to how to proceed for the rest of the assignment.

The public IP addresses used in this lab are owned by Cisco.

### Required Resources

• 1 PC (Windows 7 or 8 with Internet access)

In Part 1, you will determine the network and broadcast addresses, as well as the number of hosts, given an IPv4 address and subnet mask.

REVIEW: To determine the network address, perform binary ANDing on the IPv4 address using the subnet mask provided. The result will be the network address. Hint: If the subnet mask has decimal value 255 in an octet, the result will ALWAYS be the original value of that octet. If the subnet mask has decimal value 0 in an octet, the result will ALWAYS be 0 for that octet.

Example:

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Result (Network)            192.168.10.0

Knowing this, you may only have to perform binary ANDing on an octet that does not have 255 or 0 in its subnet mask portion.

Example:

Analyzing this example, you can see that you only have to perform binary ANDing on the third octet. The first two octets will result in 172.30 due to the subnet mask. The fourth octet will result in 0 due to the subnet mask.

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Result (Network)         172.30.?.0

Perform binary ANDing on the third octet.

Decimal               Binary

239                      11101111

192                     11000000

=======

Result    192                       11000000

Analyzing this example again produces the following result:

==========

Result (Network)         172.30.192.0

Continuing with this example, determining the number of hosts per network can be calculated by analyzing the subnet mask. The subnet mask will be represented in dotted decimal format, such as 255.255.192.0, or in network prefix format, such as /18. An IPv4 address always has 32 bits. Subtracting the number of bits used for the network portion (as represented by the subnet mask) gives you the number of bits used for hosts.

Using our example above, the subnet mask 255.255.192.0 is equivalent to /18 in prefix notation. Subtracting 18 network bits from 32 bits results in 14 bits left for the host portion. From there, it is a simple calculation:

2(number of host bits) – 2 = Number of hosts

214 = 16,384 – 2 = 16,382 hosts

Determine the network and broadcast addresses and number of host bits and hosts for the given IPv4 addresses and prefixes in the following table.

 IPv4 Address/Prefix Network Address Broadcast Address Total Number of Host Bits Total Number of Hosts 192.168.100.25/28 192.168.100.16 192.168.100.31 4 14 172.30.10.130/30 172.30.10.128 172.30.10.131 2 2 10.1.113.75/19 10.1.96.0 10.1.127.255 13 8190 198.133.219.250/24 198.133.219.0 198.133.219.255 8 254 128.107.14.191/22 128.107.12.0 128.107.15.255 10 1022 172.16.104.99/27 172.16.104.96 172.16.104.127 5 30

### Part 2: Calculate IPv4 Address Subnetting

When given an IPv4 address, the original subnet mask and the new subnet mask, you will be able to determine:

• Network address of this subnet
• Range of host addresses of this subnet
• Number of subnets created
• Number of hosts per subnet

The following example shows a sample problem along with the solution for solving this problem:

 Given: Host IP Address: 172.16.77.120 Original Subnet Mask 255.255.0.0 New Subnet Mask: 255.255.240.0 Find: Number of Subnet Bits 4 Number of Subnets Created 16 Number of Host Bits per Subnet 12 Number of Hosts per Subnet 4,094 Network Address of this Subnet 172.16.64.0 IPv4 Address of First Host on this Subnet 172.16.64.1 IPv4 Address of Last Host on this Subnet 172.16.79.254 IPv4 Broadcast Address on this Subnet 172.16.79.255

Let’s analyze how this table was completed.

The original subnet mask was 255.255.0.0 or /16. The new subnet mask is 255.255.240.0 or /20. The resulting difference is 4 bits. Because 4 bits were borrowed, we can determine that 16 subnets were created because 24 = 16.

The new mask of 255.255.240.0 or /20 leaves 12 bits for hosts. With 12 bits left for hosts, we use the following formula: 212 = 4,096 – 2 = 4,094 hosts per subnet.

Binary ANDing will help you determine the subnet for this problem, which results in the network 172.16.64.0.

Finally, you need to determine the first host, last host, and broadcast address for each subnet. One method to determine the host range is to use binary math for the host portion of the address. In our example, the last 12 bits of the address is the host portion. The first host would have all significant bits set to zero and the least significant bit set to 1. The last host would have all significant bits set to 1 and the least significant bit set to 0. In this example, the host portion of the address resides in the 3rd and 4th octets.

 Description 1st Octet 2nd Octet 3rd Octet 4th Octet Description Network/Host nnnnnnnn nnnnnnnn nnnnhhhh hhhhhhhh Subnet Mask Binary 10101100 00010000 01000000 00000001 First Host Decimal 172 16 64 1 First Host Binary 10101100 00010000 01001111 11111110 Last Host Decimal 172 16 79 254 Last Host Binary 10101100 00010000 01001111 11111111 Broadcast Decimal 172 16 79 255 Broadcast

1. Problem 1:
 Given: Host IP Address: 192.168.200.139 Original Subnet Mask 255.255.255.0 New Subnet Mask: 255.255.255.224 Find: Number of Subnet Bits 3 Number of Subnets Created 8 Number of Host Bits per Subnet 5 Number of Hosts per Subnet 30 Network Address of this Subnet 192.168.200.128 IPv4 Address of First Host on this Subnet 192.168.200.129 IPv4 Address of Last Host on this Subnet 192.168.200.158 IPv4 Broadcast Address on this Subnet 192.168.200.159
2. Problem 2:
 Given: Host IP Address: 10.101.99.228 Original Subnet Mask 255.0.0.0 New Subnet Mask: 255.255.128.0 Find: Number of Subnet Bits 9 Number of Subnets Created 512 Number of Host Bits per Subnet 15 Number of Hosts per Subnet 32,766 Network Address of this Subnet 10.101.0.0 IPv4 Address of First Host on this Subnet 10.101.0.1 IPv4 Address of Last Host on this Subnet 10.101.127.254 IPv4 Broadcast Address on this Subnet 10.101.127.255
3. Problem 3:
 Given: Host IP Address: 172.22.32.12 Original Subnet Mask 255.255.0.0 New Subnet Mask: 255.255.224.0 Find: Number of Subnet Bits 3 Number of Subnets Created 8 Number of Host Bits per Subnet 13 Number of Hosts per Subnet 8,190 Network Address of this Subnet 172.22.32.0 IPv4 Address of First Host on this Subnet 172.22.32.1 IPv4 Address of Last Host on this Subnet 172.22.63.254 IPv4 Broadcast Address on this Subnet 172.22.63.255
4. Problem 4:
 Given: Host IP Address: 192.168.1.245 Original Subnet Mask 255.255.255.0 New Subnet Mask: 255.255.255.252 Find: Number of Subnet Bits 6 Number of Subnets Created 64 Number of Host Bits per Subnet 2 Number of Hosts per Subnet 2 Network Address of this Subnet 192.168.1.244 IPv4 Address of First Host on this Subnet 192.168.1.245 IPv4 Address of Last Host on this Subnet 192.168.1.246 IPv4 Broadcast Address on this Subnet 192.168.1.247
5. Problem 5:
 Given: Host IP Address: 128.107.0.55 Original Subnet Mask 255.255.0.0 New Subnet Mask: 255.255.255.0 Find: Number of Subnet Bits 8 Number of Subnets Created 256 Number of Host Bits per Subnet 8 Number of Hosts per Subnet 254 Network Address of this Subnet 128.107.0.0 IPv4 Address of First Host on this Subnet 128.107.0.1 IPv4 Address of Last Host on this Subnet 128.107.0.254 IPv4 Broadcast Address on this Subnet 128.107.0.255
6. Problem 6:
 Given: Host IP Address: 192.135.250.180 Original Subnet Mask 255.255.255.0 New Subnet Mask: 255.255.255.248 Find: Number of Subnet Bits 5 Number of Subnets Created 32 Number of Host Bits per Subnet 3 Number of Hosts per Subnet 6 Network Address of this Subnet 192.135.250.176 IPv4 Address of First Host on this Subnet 192.135.250.177 IPv4 Address of Last Host on this Subnet 192.135.250.182 IPv4 Broadcast Address on this Subnet 192.135.250.183

### Reflection

Why is the subnet mask so important when analyzing an IPv4 address?

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The subnet mask determines everything about the address: the network, number of host bits, number of hosts and the broadcast address. Merely looking at an IPv4 address tells you nothing. You need the subnet mask to fill in all the important pieces of information.