If you’re working in networking, cybersecurity, or even basic IT administration, understanding subnetting is essential. One of the most common subnet masks you’ll encounter is “/25,” but many are unsure about how many IP addresses in a /25 exist. This question arises when segmenting networks for improved performance, security, or organization.
In IPv4, IP addresses are categorized and distributed with great precision, and subnetting allows these addresses to be logically segmented for better control. The “/25” subnet mask plays a pivotal role in this division, especially in Class C IP networks where resources are limited.
The importance of knowing how many IP addresses are in a /25 goes beyond academic curiosity. It influences how you allocate resources, configure DHCP scopes, avoid IP conflicts, and optimize network traffic. Whether you’re preparing for a certification or troubleshooting enterprise routing issues, mastering this specific IP question is crucial.
Let’s dive deep into the detailed mechanics of subnetting, understand what a /25 means, and answer the critical question: how many IP addresses in a /25 are usable, reserved, or wasted?
What Does /25 Mean in IP Addressing?
In IPv4 addressing, the “/25” notation represents the subnet mask, specifically 255.255.255.128. This subnet mask indicates that the first 25 bits are used for the network portion, leaving 7 bits for host identification. Because each bit represents a binary value of either 0 or 1, 7 bits allow for 2^7 or 128 total IP addresses.
But out of these 128, only 126 are usable for assigning to hosts. Why? Because one address is reserved as the network address (first IP in the range), and one as the broadcast address (last IP in the range). These addresses serve vital roles within the subnet—broadcasting packets to all hosts or identifying the subnet itself.
The use of /25 subnetting is common when you want to divide a traditional Class C network (which offers 256 addresses) into two equal parts. Each part will be a /25 subnet, giving you control over routing, segmentation, and traffic management.
Understanding how many IP addresses in a /25 subnet is just one step. You also need to understand how it fits into real-world configurations. For instance, in DHCP settings, you may define a range of 192.168.1.1 to 192.168.1.126, where 192.168.1.0 is the network ID and 192.168.1.127 is the broadcast address.
The question “how many IP addresses in a /25” also matters when configuring VLANs, static IP assignments, or IPAM tools. In tightly controlled or segmented environments—such as corporate LANs or cloud VPCs—precision in addressing is non-negotiable.
Moreover, knowledge of subnetting improves your chances of passing IT certification exams like CCNA, Network+, and CompTIA. Understanding how many IP addresses in a /25 means you’re familiar with binary calculation, subnet chart reading, and route planning.
In short, a /25 subnet offers an efficient way to create two 126-host networks from one 254-host Class C block. It’s practical, efficient, and often necessary in scaled-down enterprise or campus environments.
When Should You Use a /25 Subnet?
A /25 subnet is a smart choice when you need to divide a network efficiently while maintaining control and optimizing performance. Here’s when to use it:
Splitting a Class C Network Efficiently
A /25 subnet is often used when you want to divide a standard Class C network (which traditionally includes 256 IP addresses) into two equally sized subnets. Each /25 provides 128 total IP addresses, with 126 usable for hosts after subtracting the network and broadcast addresses. This is a great choice when you need logical network segmentation without wasting unused IPs.
Ideal for Medium-Sized Office Networks
For office environments that support up to 100–120 devices, a /25 subnet is typically more than sufficient. It allows network administrators to allocate IPs efficiently, especially in spaces where the number of connected devices is moderate but organization is still essential. This keeps the IP pool manageable and avoids unnecessary complexity.
Reducing Broadcast Traffic for Better Performance
One of the key advantages of subnetting is reducing the size of the broadcast domain. A /25 subnet minimizes the number of devices affected by broadcast traffic, helping to improve overall network performance and reduce congestion. This is particularly beneficial in networks with devices that generate frequent ARP or discovery broadcasts.
Useful in Cloud and Virtual Private Cloud (VPC) Deployments
In platforms like AWS, Azure, or Google Cloud, creating a /25 subnet offers administrators a clean, controlled range of IPs to assign to instances or services. It provides flexibility without overwhelming the routing setup, making it easier to scale within isolated segments.
Enhancing Routing and Firewall Configuration
Using a /25 subnet can also simplify router and firewall rule management. By segmenting traffic more narrowly, you can apply specific rules to smaller groups of devices, reducing overhead and improving security posture. Smaller subnets also result in more concise and efficient routing tables.
Why Is the Total IP Count Not Fully Usable?
A /25 subnet has 128 total IP addresses, but you can’t use them all. Here’s why:
- Network Address (1 IP): The very first address (e.g., 192.168.1.0) identifies the subnet itself and cannot be assigned to any host.
- Broadcast Address (1 IP): The last address (e.g., 192.168.1.127) sends packets to all devices in the subnet.
- Usable Host IPs (126 IPs): These are the assignable addresses ranging from 192.168.1.1 to 192.168.1.126.
- Reserved IPs: Some networks may manually reserve certain addresses for gateways, DNS, or infrastructure devices.
- Avoiding Overlaps: In large environments, overlapping subnets are a risk. Precise address planning ensures this doesn’t happen.
How to Calculate How Many IP Addresses in a /25
To understand how many IP addresses are in a /25 subnet, you need to dive into binary subnetting and the logic behind bit division. The “/25” tells us that 25 bits are fixed for the network, leaving 7 bits for the host.
With 7 host bits:
- 2^7 = 128 total addresses
- 128 – 2 = 126 usable host addresses
The two subtracted are for the network and broadcast addresses.
This subnet mask (255.255.255.128) essentially divides a Class C block (192.168.1.0/24) into two:
- Subnet 1: 192.168.1.0 to 192.168.1.127
- Subnet 2: 192.168.1.128 to 192.168.1.255
Each of these ranges is/25 and contains 128 IPs total. This is highly useful in structured IP allocation.
Subnet calculators and tools like ipcalc or subnetonline.com help automate this process, but knowing the math is essential for exams and manual configurations.
This structure ensures balanced segmentation and effective use of available IP space in networks where 256 IPs are overkill, but 64 are not enough.
How Many IP Addresses in a /25 and Its Real-World Applications
A /25 subnet offers 128 total IP addresses, making it perfect for various real-world networking scenarios. Here’s where it’s commonly applied:
Business Networks with Moderate Device Counts
A /25 is ideal for offices or departments with under 130 devices.
VOIP and Surveillance Systems
Segregating VOIP phones or IP cameras using /25 helps isolate traffic.
University Department Subnetting
Campus networks use /25 subnets to assign a block to each faculty or lab.
Home Labs and Small ISPs
Network hobbyists and local ISPs may deploy /25 for modular design.
Internet of Things (IoT) Environments
IoT devices are usually grouped and separated into subnets like /25 to reduce noise.
Conclusion
Knowing how many IP addresses in a /25 subnet exist is essential for efficient IP address planning. Whether configuring enterprise routers, designing cloud networks, or passing a networking exam, the answer—128 total, 126 usable—is a baseline fact every network engineer should know.
Precision subnetting reduces IP waste, supports better security, enhances performance, and keeps broadcast domains under control. With IP exhaustion in IPv4, every address counts. So knowing how many IP addresses in a /25 is more than technical trivia—it’s strategic knowledge.
FAQ’s
What is the subnet mask for a /25 network?
The subnet mask for a /25 network is 255.255.255.128. It divides a standard Class C range into two equal-sized subnets, enhancing network segmentation and efficiency.
How many usable IP addresses are in a /25?
A /25 subnet offers 126 usable IP addresses, as 2 out of 128 total are reserved—one for the network address and one for broadcast purposes.
Can a /25 subnet be used for two different VLANs?
Yes, a /25 subnet can define separate VLANs, helping isolate devices logically and improve performance and security within networked environments.
How does a /25 differ from a /24 subnet?
A /24 subnet has 256 IPs with 254 usable, while a /25 has only 128 total and 126 usable, splitting the address range in half for tighter control.
Is a /25 subnet suitable for home networks?
Yes, a /25 subnet is great for tech-savvy home setups, especially when managing many smart devices, home labs, or segmented internal services.