Find Four Subnet Facts 4

 In 200-301 V1 Ch14: Analyzing IPv4 Subnets, 200-301 V1 Part 4: IPv4 Addressing, IPv4 Ch. 14 Address Drills, Questions
2

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
1 192.168.88.230 /30
2 192.168.88.230 /25
3 192.168.88.155 /28
4 192.168.88.105 /26
5 192.168.88.125 /27

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: 192.168.88.230/30.

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

Subnet ID 192.168.88.228
1st Address 192.168.88.229
Last Address 192.168.88.230
Subnet Broadcast 192.168.88.231

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 255 255 255 252
Address 192 168 88 230
Apply 255 and 0 Rules for ID 192 168 88 .
Apply 255 and 0 Rules for B’cast 192 168 88 .

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 230
Magic Number (256 – mask) 256 – 252 = 4
Magic Multiple 228
Next Multiple (Too High) 232

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

  • 232 – 1 = 231

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 255 255 255 252
Address 192 168 88 230
Complete Subnet ID 192 168 88 228
Complete Subnet B’Cast 192 168 88 231

Problem 2 Answer and Explanation

This problem begins with this address/mask: 192.168.88.230/25.

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

Subnet ID 192.168.88.128
1st Address 192.168.88.129
Last Address 192.168.88.254
Subnet Broadcast 192.168.88.255

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 255 255 255 128
Address 192 168 88 230
Apply 255 and 0 Rules for ID 192 168 88 .
Apply 255 and 0 Rules for B’cast 192 168 88 .

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 230
Magic Number (256 – mask) 256 – 128 = 128
Magic Multiple 128
Next Multiple (Too High) 256

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

  • 256 – 1 = 255

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 255 255 255 128
Address 192 168 88 230
Complete Subnet ID 192 168 88 128
Complete Subnet B’cast 192 168 88 255

Problem 3 Answer and Explanation

This problem begins with this address/mask: 192.168.88.155/28.

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

Subnet ID 192.168.88.144
1st Address 192.168.88.145
Last Address 192.168.88.158
Subnet Broadcast 192.168.88.159

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 255 255 255 240
Address 192 168 88 155
Apply 255 and 0 Rules for ID 192 168 88 .
Apply 255 and 0 Rules for B’cast 192 168 88 .

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 155
Magic Number (256 – mask) 256 – 240 = 16
Magic Multiple 144
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 255 255 255 240
Address 192 168 88 155
Complete Subnet ID 192 168 88 144
Complete Subnet B’cast 192 168 88 159

Problem 4 Answer and Explanation

This problem begins with this address/mask: 192.168.88.105/26.

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

Subnet ID 192.168.88.64
1st Address 192.168.88.65
Last Address 192.168.88.126
Subnet Broadcast 192.168.88.127

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 255 255 255 192
Address 192 168 88 105
Apply 255 and 0 Rules for ID 192 168 88 .
Apply 255 and 0 Rules for B’cast 192 168 88 .

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 105
Magic Number (256 – mask) 256 – 192 = 64
Magic Multiple 64
Next Multiple (Too High) 128

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

  • 128 – 1 = 127

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 255 255 255 192
Address 192 168 88 105
Complete Subnet ID 192 168 88 64
Complete Subnet B’cast 192 168 88 127

Problem 5 Answer and Explanation

This problem begins with this address/mask: 192.168.88.125/27.

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

Subnet ID 192.168.88.96
1st Address 192.168.88.97
Last Address 192.168.88.126
Subnet Broadcast 192.168.88.127

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 255 255 255 192
Address 192 168 88 125
Apply 255 and 0 Rules for ID 192 168 88 .
Apply 255 and 0 Rules for B’cast 192 168 88 .

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 125
Magic Number (256 – mask) 256 – 224 = 32
Magic Multiple 96
Next Multiple (Too High) 128

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

  • 128 – 1 = 127

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 255 255 255 192
Address 192 168 88 105
Complete ID 192 168 88 96
Complete Subnet B’cast 192 168 88 127
Find Four Subnet Facts 3
Analyze Existing Subnet Masks 1
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Jason Cooper

2 out of 5, coulda been worse

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Wendell Odom

You’re in the game!
Now work on improving your score… Figure out what you did wrong on those you got wrong, and you’ll do better on the next set!!

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