Find Four Subnet Facts 1
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.
Five Practice Problems (Hidden)
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 | 10.100.200.30 | /13 |
| 2 | 10.10.20.30 | /17 |
| 3 | 10.77.66.55 | /21 |
| 4 | 10.101.103.105 | /25 |
| 5 | 10.199.99.9 | /24 |
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
This problem begins with this address/mask: 10.100.200.30/13.
The first table below lists the answer, with an explanation to follow.
| Subnet ID | 10.96.0.0 |
| 1st Address | 10.96.0.1 |
| Last Address | 10.103.255.254 |
| Subnet Broadcast | 10.103.255.255 |
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 | 248 | 0 | 0 |
| Address | 10 | 100 | 200 | 30 |
| Apply 255 and 0 Rules for ID | 10 | . | 0 | 0 |
| Apply 255 and 0 Rules for B’cast | 10 | . | 255 | 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 2 |
| Address Value in that Octet | 100 |
| Magic Number (256 – mask) | 256 – 248 = 8 |
| Magic Multiple | 96 |
| Next Multiple (Too High) | 104 |
Finally, the missing value for the subnet broadcast address is the next magic multiple, minus 1. In this case, per the previous table:
- 104 – 1 = 103
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 | 248 | 0 | 0 |
| Address | 10 | 100 | 200 | 30 |
| Complete Subnet ID | 10 | 96 | 0 | 0 |
| Complete Subnet B’cast | 10 | 103 | 255 | 255 |
This problem begins with this address/mask: 10.10.20.30/17.
The first table below lists the answer, with an explanation to follow.
| Subnet ID | 10.10.0.0 |
| 1st Address | 10.10.0.1 |
| Last Address | 10.10.127.254 |
| Subnet Broadcast | 10.10.127.255 |
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 | 128 | 0 |
| Address | 10 | 10 | 20 | 30 |
| Apply 255 and 0 Rules for ID | 10 | 10 | . | 0 |
| Apply 255 and 0 Rules for B’cast | 10 | 10 | . | 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 | 20 |
| Magic Number (256 – mask) | 256 – 128 = 128 |
| Magic Multiple | 0 |
| 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 | 128 | 0 |
| Address | 10 | 10 | 20 | 30 |
| Complete Subnet ID | 10 | 10 | 0 | 0 |
| Complete Subnet B’cast | 10 | 10 | 127 | 255 |
This problem begins with this address/mask: 10.77.66.55/21.
The first table below lists the answer, with an explanation to follow.
| Subnet ID | 10.77.64.0 |
| 1st Address | 10.77.64.1 |
| Last Address | 10.77.71.254 |
| Subnet Broadcast | 10.77.71.255 |
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 | 248 | 0 |
| Address | 10 | 77 | 66 | 55 |
| Apply 255 and 0 Rules for ID | 10 | 77 | . | 0 |
| Apply 255 and 0 Rules for B’cast | 10 | 77 | . | 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 4 |
| Address Value in that Octet | 66 |
| Magic Number (256 – mask) | 256 – 248 = 8 |
| Magic Multiple | 64 |
| Next Multiple (Too High) | 72 |
Finally, the missing value for the subnet broadcast address is the next magic multiple, minus 1. In this case, per the previous table:
- 72 – 1 = 71
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 | 248 | 0 |
| Address | 10 | 77 | 66 | 55 |
| Complete Subnet ID | 10 | 77 | 64 | 0 |
| Complete Subnet B’cast | 10 | 77 | 71 | 255 |
This problem begins with this address/mask: 10.101.103.105/25.
The first table below lists the answer, with an explanation to follow.
| Subnet ID | 10.101.103.0 |
| 1st Address | 10.101.103.1 |
| Last Address | 10.101.103.126 |
| Subnet Broadcast | 10.101.103.127 |
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 | 128 |
| Address | 10 | 101 | 103 | 105 |
| Apply 255 and 0 Rules for ID | 10 | 101 | 103 | . |
| Apply 255 and 0 Rules for B’cast | 10 | 101 | 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 | 105 |
| Magic Number (256 – mask) | 256 – 128 = 128 |
| Magic Multiple | 0 |
| 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 | 128 |
| Address | 10 | 101 | 103 | 105 |
| Complete Subnet ID | 10 | 101 | 103 | 0 |
| Complete Subnet B’cast | 10 | 101 | 103 | 127 |
This problem begins with this address/mask: 10.199.99.9/24.
The first table below lists the answer, with an explanation to follow.
| Subnet ID | 10.199.99.0 |
| 1st Address | 10.199.99.1 |
| Last Address | 10.199.99.254 |
| Subnet Broadcast | 10.199.99.255 |
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 | 10 | 199 | 99 | 9 |
| Apply 255 and 0 Rules for ID | 10 | 199 | 99 | 0 |
| Apply 255 and 0 Rules for B’cast | 10 | 199 | 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!