In the realm of network engineering and administration, understanding how to efficiently subdivide IP networks into smaller, manageable segments—known as subnetting—is fundamental. With the advent of IPv4’s limited address space, subnetting has become an essential skill to optimize the use of IP addresses.
IPv4 addresses consist of 32 bits, typically represented in decimal format as four octets separated by dots (e.g., 192.168.1.1). Subnetting involves dividing a network into smaller networks (subnets) to improve network management and security. This division is achieved through the subnet mask, which determines how many bits are used for the network portion and how many for the host portion of an address.
To simplify subnetting, follow this straightforward six-step method, which can be applied to any IPv4 address:
Following this method, the subnetting details for the IP address 10.20.4.13 with a /29 prefix are as follows:
A Variable Length Subnet Mask (VLSM) allows for the subnet masks of variable lengths to be used within the same network, providing more flexibility and efficiency in IP address allocation. Here’s a brief overview:
| Prefix size | Network mask | Usable hosts per subnet |
| /1 | 128.0.0.0 | 2,147,483,646 |
| /2 | 192.0.0.0 | 1,073,741,822 |
| /3 | 224.0.0.0 | 536,870,910 |
| /4 | 240.0.0.0 | 268,435,454 |
| /5 | 248.0.0.0 | 134,217,726 |
| /6 | 252.0.0.0 | 67,108,862 |
| /7 | 254.0.0.0 | 33,554,430 |
| Class A | ||
| /8 | 255.0.0.0 | 16,777,214 |
| /9 | 255.128.0.0 | 8,388,606 |
| /10 | 255.192.0.0 | 4,194,302 |
| /11 | 255.224.0.0 | 2,097,150 |
| /12 | 255.240.0.0 | 1,048,574 |
| /13 | 255.248.0.0 | 524,286 |
| /14 | 255.252.0.0 | 262,142 |
| /15 | 255.254.0.0 | 131,070 |
| Class B | ||
| /16 | 255.255.0.0 | 65,534 |
| /17 | 255.255.128.0 | 32,766 |
| /18 | 255.255.192.0 | 16,382 |
| /19 | 255.255.224.0 | 8,190 |
| /20 | 255.255.240.0 | 4,094 |
| /21 | 255.255.248.0 | 2,046 |
| /22 | 255.255.252.0 | 1,022 |
| /23 | 255.255.254.0 | 510 |
| Class C | ||
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /26 | 255.255.255.192 | 62 |
| /27 | 255.255.255.224 | 30 |
| /28 | 255.255.255.240 | 14 |
| /29 | 255.255.255.248 | 6 |
| /30 | 255.255.255.252 | 2 |
| /31 | 255.255.255.254 | 0 (special use) |
| /32 | 255.255.255.255 | 0 (single host) |
Subnetting is a critical skill in network design and management, enabling the efficient use of IP addresses and enhanced network security. By mastering the easy six-step method outlined above, network administrators and engineers can simplify the subnetting process. Remember, practice is key to becoming proficient in subnetting and mask calculations. With these tools and methods, navigating the complexities of IPv4 networks becomes a more manageable and straightforward task.
Evgeny Sevastyanov
Client Support Teamleader
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