If you work with IPv4 networks, the question How many ip addresses in a /25? pops up often, especially when splitting a /24 or planning access segments. The short version: a /25 has 128 total addresses and typically 126 usable hosts (network and broadcast consume two). But great planning needs more than a number. You also need the subnet mask, binary boundaries, sample ranges, and when a /25 is a smart pick versus a /26 or a/24.

In this guide, we’ll nail the math behind How many ip addresses in a /25?, show how to compute it instantly from first principles, and translate the answer into practical steps you can apply in real deployments—Wi-Fi floors, lab pods, IoT segments, and DMZ slices. We’ll walk through examples (like carving/24 into two /25s), clarify special cases, and call out gotchas that bite during rollouts or audits.

The quick guide to /25 IP totals

A /25 subnet in IPv4 contains 128 addresses. That figure comes from a simple rule: IPv4 has 32 bits, so the number of host addresses in a prefix/p/p is 2^(32−p). For /25, that’s 2^(7) = 128. In regular host networks, two addresses aren’t assignable—the lowest (network) and highest (broadcast)—so you usually have 126 usable. When someone asks How many ip addresses in a /25?, this is the precise but practical answer: 128 total, 126 usable.

Consider the mask. /25 translates to the dotted decimal mask 255.255.255.128. That last octet’s 128 indicates the block size—your ranges jump by 128. If you’re splitting a /24, you get two neat halves: .0–.127 and .128–.255. That predictability is why many admins default to /25 when they want a tidy split with around a hundred hosts per segment.

Let’s ground it with an example. Take 192.168.10.0/25: network 192.168.10.0, broadcast 192.168.10.127, and usable hosts .1–.126. The “other half” of the original /24 becomes 192.168.10.128/25: network. .128, broadcas.t .255, and usabl.e .129–.254. Ask again, How many ip addresses in a /25?—you can now recite the answer and map it to actual address ranges.

Why do engineers pick /25 beyond How many ip addresses in a /25? Primarily right-sizing. /24 (254 usable) might be too roomy, encouraging sprawl and noisy broadcasts. /26 (62 usable) might be too tight for a floor of devices, printers, APs, and management IPs. /25 hits the sweet spot—enough host space with good containment for ARP and broadcast domains.

When a /25 Subnet Is the Right Fit

If you’ve just asked “How many ip addresses in a /25?”, here’s when that /25 is the right fit—and why.

Sizing after “How many ip addresses in a /25?”

Once you know How many ip addresses in a /25? (128 total, 126 usable), match it to your device count. Include users, printers, phones, access points, IoT, and a growth buffer (20–30%). If your projection lands between ~70 and ~110 devices, a /25 usually fits well.

Broadcast domain impact with a /25

/25 halves a /24’s broadcast domain. That reduces ARP chatter and helps keep latency spikes down in busy segments. If you asked How many ip addresses in a /25? to limit noise, this is one win you’ll feel on high-density floors.

Address management trade-offs

Two /25s from a /24 keep routing simple and carve clean ranges. They’re easy to document and audit. If you frequently reorganize VLANs, the /25 pattern is a safe building block because the answer to How many ip addresses in a /25? remains constant across floors or pods.

Growth & futureproofing

What if you outgrow a /25? Plan migration paths: reserve adjacent space, use DHCP scopes smartly, and track utilization. The question How many ip addresses in a /25? should be paired with What’s our 12-month headroom?

Security segmentation

A /25 makes it easier to isolate device classes (e.g., guests vs. corp). If the conversation starts with How many ip addresses in a /25?, it often ends with how can we segment to reduce risk?

Master the /25 subnet for mid-sized networks

These special IPv4 cases affect how addresses are consumed on a link, but they don’t change that a /25 has 128 total (126 usable).

• /31 point-to-point links (RFC 3021). No network or broadcast; both addresses are usable. Ideal for router interconnects and tunnels, saving space compared with /30.

• /32 loopbacks. One-address interfaces are used for management, router IDs, and services. They’re never host segments and don’t affect LAN usable-address math.

• Legacy /30 on P2P. Has a network and broadcast, so only 2 are usable. Still appears for compatibility/compliance, but it’s wasteful next to /31.

• Virtual gateways (VRRP/HSRP/GLBP). A virtual IP consumes one address for redundancy. The theoretical /25 count stays 126 usable, but the practical host capacity drops by one.

• DHCP realities. Reservations, exclusions, and fixed infrastructure (printers, APs, cameras) shrink the pool you can hand to end hosts. Document these early so utilization reports don’t surprise you.

• NAT and CGNAT. Translation eases public scarcity but doesn’t expand the L2 subnet. The on-link addressing math remains unchanged.

Practical /25 Subnetting for Network Design

Let’s take 10.20.30.0/24 and split it. The first half is 10.20.30.0/25 with 128 addresses. Network is 10.20.30.0, broadcast 10.20.30.127, and hosts .1–.126. The second half is 10.20.30.128/25 with network .128, broadcast .255, and hosts .129–.254. If a team lead asks, “How many ip addresses in a /25?”, you can answer 128 total and immediately show both halves—no calculator needed. 

Now put it in a building. Floor 3 has ~90 devices. /26 (62 usable) fails. /24 (254 usable) is overkill and noisier. Because we know How many ip addresses in a /25?, we pick /25 and assign 10.20.30.0/25 to Floor 3. Floor 4 gets 10.20.30.128/25. Documentation stays clean: identical size, neat masks, predictable ranges. 

Or think Wi-Fi. Each AP needs a management IP; clients live elsewhere, but staff add printers and phones over time. The team keeps asking How many ip addresses in a /25? to check headroom. With ~70-80 active devices and planned growth, /25 hits the target.
From a security view, two /25s simplify ACLs. You can write rules per half of the original /24. When someone new joins and asks How many ip addresses in a /25?, your diagrams and ranges are so consistent that the number 128 anchors the whole discussion.

The /25 Playbook for Clean IP Planning

Here’s a quick planning cheat sheet that links mask to math so you can answer “How many ip addresses in a /25?” at a glance.

25 subnet size and mask (255.255.255.128)

The mask tells the story. 255.255.255.128 equals /25. Ask How many ip addresses in a /25? and you also imply the mask; they’re two faces of the same coin.

The formula behind “How many ip addresses in a /25?”

2^(32−25) = 2⁷ = 128. Remove network and broadcast for host assignments → 126 usable.

24 split into two /25s for clean ranges

Two symmetrical halves: x.x.x.0/25 and x.x.x.128/25. This is the fastest way to operationalize How many ip addresses in a /25? in a campus.

When a/25/25 beats/26 or /24

If broadcast noise matters and you need ~70–110 hosts, /25 is the right-size subnet. The question How many ip addresses in a /25? feeds directly into this choice.

Conclusion

If you’re thinking, “How many addresses are in a /25 subnet?” the correct, actionable result is 128 total addresses, 126 typically usable with a mask of 255.255.255.128. That single fact unlocks fast splits of/24, cleaner broadcast domains, and predictable documentation. The next time someone asks How many ip addresses in a /25?, you won’t just give a number—you’ll guide the design choice that fits today’s load and tomorrow’s growth.

FAQs

What is the exact answer to “How many ip addresses in a /25?”
128 total, usually 126 usable (subtract network and broadcast). Mask 255.255.255.128.

Why subtract two after “How many ip addresses in a /25?”
In standard IPv4 subnets, the lowest address is the network ID, and the highest isthe broadcast. Hosts shouldn’t use those.

Can I use all 128 in a /25?
Only in special cases without broadcast (not typical for access LANs). For normal user/device subnets, expect 126 usable.

Is /25 good for Wi-Fi or office floors?
Yes—if you need ~70–110 hosts. Knowing How many ip addresses in a /25? helps right-size without a noisy /24.

How do I split a /24 into two /25s?
Take x.x.x.0/24 and produce x.x.x.0/25 and x.x.x.128/25. You’ll have two blocks of 128 addresses each.