The Complete Guide Subnet Masks Frome Beginner’s Guide
A subnet mask is an essential part of networking that helps divide IP addresses into smaller, manageable groups. Here’s a detailed step-by-step explanation of subnet masks, how they work, and how to calculate and use them, providing a clear understanding of this fundamental networking concept.
What Is a Subnet Mask?
A subnet mask is a 32-bit value used in IPv4 networking that distinguishes the portion of an IP address that represents the network from the portion that refers to the host within the network. It is usually written in a format identical to an IP address, such as 255.255.255.0. The subnet mask works like a filter: by aligning IP addresses with a mask, devices can instantly determine whether another address is inside their local network or on a remote network, which is essential for packet routing decisions.
How Subnet Masks Work
The subnet mask uses binary numbers, where ones (1s) indicate the network portion and zeros (0s) indicate the host portion. For example, the mask 255.255.255.0, in binary, looks like 11111111.11111111.11111111.00000000, indicating that the first 24 bits of the address refer to the network and the last eight to hosts. When a device needs to send data, it compares its own IP address with the target’s using the subnet mask to determine if the recipient is local or remote.
Subnet Mask and CIDR Notation
CIDR (Classless Inter-Domain Routing) notation shortens the subnet mask using a slash followed by the number of bits set to 1 in the mask (for example, /24 for 255.255.255.0). This notation aids both human readability and network calculations, especially in complex IP addressing schemes often required by large organizations.
Why Use Subnet Masks?
Subnet masks offer a range of significant advantages:
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Network Segmentation: Large networks can be split into smaller subnets, improving performance and security by isolating traffic and reducing congestion.
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Efficient Address Management: Subnetting enables more effective use of finite IP address space, preventing wastage and supporting a greater number of separate networks and devices.
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Facilitates Routing: Routers use subnet masks to decide where to forward data packets, enabling efficient and organized data transmission across interconnected networks.
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Security: Restricting broadcast domains and reducing visibility of hosts improves network security.
Subnet Mask Calculation Example
To illustrate subnet mask calculation, consider the typical network 192.168.1.0/26:
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The CIDR /26 indicates 26 network bits, leaving 6 bits for hosts.
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In binary, the mask is 11111111.11111111.11111111.11000000, or 255.255.255.192 in decimal.
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This subnet accommodates 62 usable host IP addresses (2^6 – 2 = 62, excluding the network and broadcast addresses).
Subnet Mask in IPv6
While subnet masks are foundational to IPv4 subnetting, IPv6 uses CIDR-like prefix lengths to serve the same function, though the principle—separating network from host portions—remains consistent.
Key Subnet Mask Values
| Subnet Mask | CIDR | Usable Hosts | Common Use |
|---|---|---|---|
255.0.0.0 |
/8 |
16,777,214 |
Large networks |
255.255.0.0 |
/16 |
65,534 |
Campus LANs |
255.255.255.0 |
/24 |
254 |
Small LANs |
255.255.255.192 |
/26 |
62 |
Small segments |
Step 1: Understand IP Addresses and Binary Format
IP addresses are a set of four numbers (octets) separated by dots, like 192.168.0.1. Each octet is a number between 0 and 255 and corresponds to 8 bits, making a total of 32 bits. These bits are the core data that computers use to communicate.
Subnet masks are also written like IP addresses, e.g., 255.255.255.0, and in binary, the mask contains a series of 1s followed by 0s. The 1s mask the network part, and the 0s represent hosts. For example:
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255.255.255.0 in binary: 11111111.11111111.11111111.00000000
This means the first 24 bits are the network, and the last 8 bits are used for hosts.
Step 2: Why Do We Use Subnet Masks?
Without subnet masks, networks would be flat, meaning all devices would be in one large network, leading to congestion, security risks, and inefficient management. Subnet masks allow networks to be broken down into smaller subnetworks or subnets, each with its own range of IP addresses, allowing:
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Improved network performance by limiting traffic to smaller groups
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Enhanced security by isolating subnet traffic
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Efficient IP address management to reduce wastage
Step 3: How to Identify Network and Host Portions Using a Subnet Mask
To find out which part of an IP address is the network and which part is the host, the subnet mask is applied to the IP address through a bitwise AND operation. This means comparing the IP and mask bits:
Example:
IP Address: 192.168.1.10 → 11000000.10101000.00000001.00001010
Subnet Mask: 255.255.255.0 → 11111111.11111111.11111111.00000000
Applying AND operation, the network is the part where both IP and mask have 1s:
Network Address: 192.168.1.0 → 11000000.10101000.00000001.00000000
The remaining bits (zeros in the mask) are for hosts.
Step 4: Calculate the Number of Hosts Per Subnet
The number of available hosts in a subnet depends on how many bits are zeros in the subnet mask. The formula to calculate usable hosts is:
Usable Hosts=2h−2
where h = number of host bits (zeros in the mask). The subtraction of two accounts for the network address (first address) and the broadcast address (last address), which cannot be assigned to devices.
Example:
For subnet mask 255.255.255.0:
Host bits = 8 (since last octet is 00000000), so:
28−2=256−2=254 usable hosts
Step 5: Learn CIDR Notation
CIDR (Classless Inter-Domain Routing) notation is a shorthand way to write subnet masks by counting the number of 1s in the mask.
Example:
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255.255.255.0 = /24 (since 24 bits are 1)
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255.255.254.0 = /23 (23 bits are 1)
CIDR simplifies notation and helps networking professionals quickly understand subnet size and range.
Step 6: Calculate an Appropriate Subnet Mask
To set up subnets based on the number of hosts or subnets needed, subnet masks are calculated stepwise:
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Hosts-based approach: Determine the required host count, then find the smallest number of host bits h to satisfy it.
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Subnets-based approach: Determine how many subnets are needed and add enough bits for subnetting.
Example:
If you need 50 hosts per subnet:
2h−2≥50
Try h=6 (since 26−2=62), so 6 bits for hosts and remaining bits for the network.
Step 7: Understanding Variable Length Subnet Masks (VLSM)
VLSM allows for different subnet sizes within the same network, providing flexibility. Large subnets can be broken into smaller parts without wasted IP addresses:
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Use longer masks for small subnets (fewer hosts)
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Use shorter masks for larger subnets (more hosts)
This technique optimizes IP allocation and network performance.
Step 8: Practical Subnetting Example
Consider the network 192.168.1.0 with a subnet mask of 255.255.255.192 (/26):
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In binary, mask is: 11111111.11111111.11111111.11000000
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Number of host bits = 6 (last 6 bits 0)
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Usable hosts = 26−2=62
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Number of subnets from borrowing 2 bits for subnetting from the last octet: 22=4 subnets
This results in four subnets each with up to 62 hosts, perfect for dividing a medium-sized network.
Step 9: Summary and Benefits
Using subnet masks:
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Helps organize large networks into smaller, manageable parts
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Improves network efficiency and security
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Ensures optimal use of IP addresses
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Essential for routing decisions