ITN 101 Subnetting Shortcuts
Subnetting Shortcuts Study Guide
Key Concepts
Subnet Masks: Defines the range of IP addresses that can be used within a subnet. Common masks include:
/24 (255.255.255.0)
/16 (255.255.0.0)
/8 (255.0.0.0)
CIDR Notation: Represents the number of bits allocated for the network portion of the address. The format is IP Address/Prefix Length (e.g., 192.168.1.0/24).
Calculation Shortcuts
Finding Subnet Mask: Use the formula: /n (where n is the number of bits for the network):Example: /24 = 255.255.255.0
Determining Number of Hosts: Use the formula:Number of Hosts = 2^(32 - subnet bits) - 2Example: /24 provides 2^(32-24) - 2 = 254 hosts
Subnet Size: The subnet size can be calculated as:Size = 2^(number of host bits)Example: For /24 (8 host bits), Size = 2^8 = 256
Common Subnetting Shortcuts
Classful Addressing:
Class A: 0-127 (255.0.0.0)
Class B: 128-191 (255.255.0.0)
Class C: 192-223 (255.255.255.0)
Quick Reference for Common CIDR Ranges:
/30 : 4 IPs (2 usable)
/29 : 8 IPs (6 usable)
/28 : 16 IPs (14 usable)
/27 : 32 IPs (30 usable)
/26 : 64 IPs (62 usable)
/25 : 128 IPs (126 usable)
/24 : 256 IPs (254 usable)
/23 : 512 IPs (510 usable)
/22 : 1024 IPs (1022 usable)
Magic Number Calculation
Understanding subnetting is crucial for checking configurations, estimating new setups, and answering questions like those on the CompTIA Network+ exam.
A key subnetting formula involves the magic number. The magic number can be calculated in two ways:
Subtract the unusual octet in the subnet mask from 256.
Determine the number of host bits, then compute 2 to the power of that number (e.g., if 27 bits are for the network, 5 bits for hosts: 2^5 = 32).
Using the Magic Number:
You can calculate each subnet's network address by adding the magic number to the last octet. For instance, starting from 192.168.0.0 with a magic number of 32 gives you 192.168.0.32, 192.168.0.64, etc.
You can also use the magic number to find subnet IDs and the maximum hosts in each subnet (magic number - 2).
Shortcut Tool for Subnetting
Create three rows of numbers:
Start with 1, doubling left to 128.
Write 255 on the right and subtract the upper row each time.
The top row indicates the number of subnets; the middle gives the magic number; the bottom row reveals the interesting octet in the subnet mask.
Practice Problems:
To find sufficient subnets for a requirement (e.g., 28 subnets), identify the nearest power of 2 (32) and use its magic number (8) and interesting octet (248).
For subnets supporting 50 hosts, find the next largest number (64) to derive 4 subnets with 64 addresses each.
Tips for Fast Calculations
Memorize the binary equivalents of decimal numbers.
Use the formula relations to
The binary equivalents of decimal numbers are based on the powers of 2, where each decimal digit is represented by a combination of 0s and 1s. Here are some common decimal numbers and their binary equivalents:
0 = 0000
1 = 0001
2 = 0010
3 = 0011
4 = 0100
5 = 0101
6 = 0110
7 = 0111
8 = 1000
9 = 1001
10 = 1010
11 = 1011
12 = 1100
13 = 1101
14 = 1110
15 = 1111
16 = 10000
This pattern continues, increasing in similar fashion for larger decimal numbers.
ITN 101 Subnetting Shortcuts
Subnetting Shortcuts Study Guide
Key Concepts
Subnet Masks: Defines the range of IP addresses that can be used within a subnet. Common masks include:
/24 (255.255.255.0)
/16 (255.255.0.0)
/8 (255.0.0.0)
CIDR Notation: Represents the number of bits allocated for the network portion of the address. The format is IP Address/Prefix Length (e.g., 192.168.1.0/24).
Calculation Shortcuts
Finding Subnet Mask: Use the formula: /n (where n is the number of bits for the network):Example: /24 = 255.255.255.0
Determining Number of Hosts: Use the formula:Number of Hosts = 2^(32 - subnet bits) - 2Example: /24 provides 2^(32-24) - 2 = 254 hosts
Subnet Size: The subnet size can be calculated as:Size = 2^(number of host bits)Example: For /24 (8 host bits), Size = 2^8 = 256
Common Subnetting Shortcuts
Classful Addressing:
Class A: 0-127 (255.0.0.0)
Class B: 128-191 (255.255.0.0)
Class C: 192-223 (255.255.255.0)
Quick Reference for Common CIDR Ranges:
/30 : 4 IPs (2 usable)
/29 : 8 IPs (6 usable)
/28 : 16 IPs (14 usable)
/27 : 32 IPs (30 usable)
/26 : 64 IPs (62 usable)
/25 : 128 IPs (126 usable)
/24 : 256 IPs (254 usable)
/23 : 512 IPs (510 usable)
/22 : 1024 IPs (1022 usable)
Magic Number Calculation
Understanding subnetting is crucial for checking configurations, estimating new setups, and answering questions like those on the CompTIA Network+ exam.
A key subnetting formula involves the magic number. The magic number can be calculated in two ways:
Subtract the unusual octet in the subnet mask from 256.
Determine the number of host bits, then compute 2 to the power of that number (e.g., if 27 bits are for the network, 5 bits for hosts: 2^5 = 32).
Using the Magic Number:
You can calculate each subnet's network address by adding the magic number to the last octet. For instance, starting from 192.168.0.0 with a magic number of 32 gives you 192.168.0.32, 192.168.0.64, etc.
You can also use the magic number to find subnet IDs and the maximum hosts in each subnet (magic number - 2).
Shortcut Tool for Subnetting
Create three rows of numbers:
Start with 1, doubling left to 128.
Write 255 on the right and subtract the upper row each time.
The top row indicates the number of subnets; the middle gives the magic number; the bottom row reveals the interesting octet in the subnet mask.
Practice Problems:
To find sufficient subnets for a requirement (e.g., 28 subnets), identify the nearest power of 2 (32) and use its magic number (8) and interesting octet (248).
For subnets supporting 50 hosts, find the next largest number (64) to derive 4 subnets with 64 addresses each.
Tips for Fast Calculations
Memorize the binary equivalents of decimal numbers.
Use the formula relations to
The binary equivalents of decimal numbers are based on the powers of 2, where each decimal digit is represented by a combination of 0s and 1s. Here are some common decimal numbers and their binary equivalents:
0 = 0000
1 = 0001
2 = 0010
3 = 0011
4 = 0100
5 = 0101
6 = 0110
7 = 0111
8 = 1000
9 = 1001
10 = 1010
11 = 1011
12 = 1100
13 = 1101
14 = 1110
15 = 1111
16 = 10000
This pattern continues, increasing in similar fashion for larger decimal numbers.
