Find Four Subnet Facts 3

 In 200-301 V1 Ch14: Analyzing IPv4 Subnets, 200-301 V1 Part 4: IPv4 Addressing, IPv4 Ch. 14 Address Drills, Questions
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These practice posts focus on the single most important subnetting process: Find the four defining facts about a subnet based on one address and mask that resides in that network. Your first goal should be to get the right answer consistently. For those who care to pass Cisco certifications, you should strive to get the answer to each problem in about 30 seconds. So, get a stopwatch or clock app open and get ready to practice!

Videos: How to and More Practice

The left video discusses the IP subnet concepts, while the center video focuses on the decimal process to find the four values that define a subnet. The video on the right provides a few practice problems with a demonstration of how to use the decimal process.

Subnetting Explained!

How to Find Subnet Facts!

Practice: Find Subnet Facts

Five Practice Problems (Hidden)

Problems (Hidden) - Get Your Stopwatch Ready!

For each problem below, derive the four key facts about the related subnet:

  • Subnet ID
  • First (Lowest) Usable IP Address
  • Last (Highest) Usable IP Address
  • Subnet Broadcast Address
Problem IP Address Mask
1 172.16.155.44 /19
2 172.16.18.109 /29
3 172.16.211.14 /20
4 172.16.103.81 /27
5 172.16.99.99 /24

Magic Multiple Reference

The decimal process asks you to calculate a magic number and then pick two multiples of the magic number. When the address happens to use higher numbers in the IPv4 address legal range (0-255 inclusive), finding the magic multiple might consume more time than you would like to take. Practice with this table that lists the multiples of 16, 32, and 64: Some of the more difficult magic numbers to work with.

Multiples of 16 Multiples of 32 Multiples of 64
0 0 0
16 . .
32 32 .
48 . .
64 64 64
80 . .
96 96 .
112 . .
128 128 128
144 . .
160 160 .
176 . .
192 192 192
208 . .
224 224 .
240 . .

Answers and Explanations for Each Problem

Problem 1 Answer and Explanation

This problem begins with this address/mask: 172.16.155.44/19.

The first table below lists the answer, with an explanation to follow.

Subnet ID 172.16.128.0
1st Address 172.16.128.1
Last Address 172.16.159.254
Subnet Broadcast 172.16.159.255

If using the decimal process detailed in the CCNA OCGs, my YouTube videos, and other subnetting products, first focus on finding the subnet ID and subnet broadcast address. To begin that process:

  • Record the DDN mask first, followed by the address.
  • Apply the “255” and “0” rules, based on the value in each octet of the mask, to begin building the subnet ID and subnet broadcast addresses. The rules:
    • Subnet ID: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 0. Otherwise, leave the octet blank for now.
    • Subnet Broadcast Address: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 255. Otherwise, leave the octet blank for now.

For this problem:

Notes Octet 1 Octet 2 Octet 3 Octet 4
DDN Mask 255 255 224 0
Address 172 16 155 44
Apply 255 and 0 Rules for ID 172 16 . 0
Apply 255 and 0 Rules for B’cast 172 16 . 255

The subnet ID’s missing octet is a multiple of the magic number: The multiple of the magic number closest to the address’s value in that octet – without being higher than the address’s value. The magic number is 256 minus the mask’s value in the remaining octet. In this case:

Missing Octet Octet 3
Address Value in that Octet 155
Magic Number (256 – mask) 256 – 224 = 32
Magic Multiple 128
Next Multiple (Too High) 160

Finally, the missing value for the subnet broadcast address is the next magic multiple, minus 1. In this case, per the previous table:

  • 160 – 1 = 159

Substituting the values into the final octets, you should find the following values for this problem:

Notes Octet 1 Octet 2 Octet 3 Octet 4
DDN Mask 255 255 224 0
Address 172 16 155 44
Complete Subnet ID 172 16 128 0
Complete Subnet B’cast 172 16 159 255

Problem 2 Answer and Explanation

This problem begins with this address/mask: 172.16.18.109/29.

The first table below lists the answer, with an explanation to follow.

Subnet ID 172.16.18.104
1st Address 172.16.18.105
Last Address 172.16.18.110
Subnet Broadcast 172.16.18.111

If using the decimal process detailed in the CCNA OCGs, my YouTube videos, and other subnetting products, first focus on finding the subnet ID and subnet broadcast address. To begin that process:

  • Record the DDN mask first, followed by the address.
  • Apply the “255” and “0” rules, based on the value in each octet of the mask, to begin building the subnet ID and subnet broadcast addresses. The rules:
    • Subnet ID: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 0. Otherwise, leave the octet blank for now.
    • Subnet Broadcast Address: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 255. Otherwise, leave the octet blank for now.

For this problem:

Notes Octet 1 Octet 2 Octet 3 Octet 4
DDN Mask 255 255 255 248
Address 172 16 18 109
Apply 255 and 0 Rules for ID 172 16 18 .
Apply 255 and 0 Rules for B’cast 172 16 18 .

 

The subnet ID’s missing octet is a multiple of the magic number: The multiple of the magic number closest to the address’s value in that octet – without being higher than the address’s value. The magic number is 256 minus the mask’s value in the remaining octet. In this case:

Missing Octet Octet 4
Address Value in that Octet 109
Magic Number (256 – mask) 256 – 248 = 8
Magic Multiple 104
Next Multiple (Too High) 112

Finally, the missing value for the subnet broadcast address is the next magic multiple, minus 1. In this case, per the previous table:

  • 112 – 1 = 111

Substituting the values into the final octets, you should find the following values for this problem:

Notes Octet 1 Octet 2 Octet 3 Octet 4
DDN Mask 255 255 255 248
Address 172 16 18 109
Complete Subnet ID 172 16 18 104
Complete Subnet B’cast 172 16 18 111

Problem 3 Answer and Explanation

This problem begins with this address/mask: 172.16.211.14/20.

The first table below lists the answer, with an explanation to follow.

Subnet ID 172.16.208.0
1st Address 172.16.208.1
Last Address 172.16.223.254
Subnet Broadcast 172.16.223.255

If using the decimal process detailed in the CCNA OCGs, my YouTube videos, and other subnetting products, first focus on finding the subnet ID and subnet broadcast address. To begin that process:

  • Record the DDN mask first, followed by the address.
  • Apply the “255” and “0” rules, based on the value in each octet of the mask, to begin building the subnet ID and subnet broadcast addresses. The rules:
    • Subnet ID: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 0. Otherwise, leave the octet blank for now.
    • Subnet Broadcast Address: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 255. Otherwise, leave the octet blank for now.

For this problem:

Notes Octet 1 Octet 2 Octet 3 Octet 4
DDN Mask 255 255 240 0
Address 172 16 211 14
Apply 255 and 0 Rules for ID 172 16 . 0
Apply 255 and 0 Rules for B’cast 172 16 . 255

The subnet ID’s missing octet is a multiple of the magic number: The multiple of the magic number closest to the address’s value in that octet – without being higher than the address’s value. The magic number is 256 minus the mask’s value in the remaining octet. In this case:

Missing Octet Octet 3
Address Value in that Octet 211
Magic Number (256 – mask) 256 – 240 = 16
Magic Multiple 208
Next Multiple (Too High) 224

Finally, the missing value for the subnet broadcast address is the next magic multiple, minus 1. In this case, per the previous table:

  • 224 – 1 = 223

Substituting the values into the final octets, you should find the following values for this problem:

Notes Octet 1 Octet 2 Octet 3 Octet 4
DDN Mask 255 255 240 0
Address 172 16 211 14
Complete Subnet ID 172 16 208 0
Complete Subnet B’cast 172 16 223 255

Problem 4 Answer and Explanation

This problem begins with this address/mask: 172.16.103.81/27.

The first table below lists the answer, with an explanation to follow.

Subnet ID 172.16.103.64
1st Address 172.16.103.65
Last Address 172.16.103.94
Subnet Broadcast 172.16.103.95

If using the decimal process detailed in the CCNA OCGs, my YouTube videos, and other subnetting products, first focus on finding the subnet ID and subnet broadcast address. To begin that process:

  • Record the DDN mask first, followed by the address.
  • Apply the “255” and “0” rules, based on the value in each octet of the mask, to begin building the subnet ID and subnet broadcast addresses. The rules:
    • Subnet ID: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 0. Otherwise, leave the octet blank for now.
    • Subnet Broadcast Address: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 255. Otherwise, leave the octet blank for now.

For this problem:

Notes Octet 1 Octet 2 Octet 3 Octet 4
DDN Mask 255 255 255 224
Address 172 16 103 81
Apply 255 and 0 Rules for ID 172 16 103 .
Apply 255 and 0 Rules for  B’cast 172 16 103 .

The subnet ID’s missing octet is a multiple of the magic number: The multiple of the magic number closest to the address’s value in that octet – without being higher than the address’s value. The magic number is 256 minus the mask’s value in the remaining octet. In this case:

Missing Octet Octet 4
Address Value in that Octet 81
Magic Number (256 – mask) 256 – 224 = 32
Magic Multiple 64
Next Multiple (Too High) 96

Finally, the missing value for the subnet broadcast address is the next magic multiple, minus 1. In this case, per the previous table:

  • 96 – 1 = 95

Substituting the values into the final octets, you should find the following values for this problem:

Notes Octet 1 Octet 2 Octet 3 Octet 4
DDN Mask 255 255 255 224
Address 172 16 103 81
Complete Subnet ID 172 16 103 64
Complete Subnet B’cast 172 16 103 95

Problem 5 Answer and Explanation

This problem begins with this address/mask: 172.16.99.99/24.

The first table below lists the answer, with an explanation to follow.

Subnet ID 172.16.99.0
1st Address 172.16.99.1
Last Address 172.16.99.254
Subnet Broadcast 172.16.99.255

If using the decimal process detailed in the CCNA OCGs, my YouTube videos, and other subnetting products, first focus on finding the subnet ID and subnet broadcast address. To begin that process:

  • Record the DDN mask first, followed by the address.
  • Apply the “255” and “0” rules, based on the value in each octet of the mask, to begin building the subnet ID and subnet broadcast addresses. The rules:
    • Subnet ID: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 0. Otherwise, leave the octet blank for now.
    • Subnet Broadcast Address: If the mask octet is 255, copy the address octet. If the mask octet is 0, write a 255. Otherwise, leave the octet blank for now.

For this problem:

Notes Octet 1 Octet 2 Octet 3 Octet 4
DDN Mask 255 255 255 0
Address 172 16 99 99
Apply 255 and 0 Rules for ID 172 16 99 0
Apply 255 and 0 Rules for B’cast 172 16 99 255

In this case, you can find all the values without the rest of the process, because all the DDN mask octets are either a 255 or a 0. You’re finished!

Find Four Subnet Facts 2
Find Four Subnet Facts 4
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