# /1 Subnet Mask (128.0.0.0) – Hosts, Range & FAQ

> The /1 subnet mask is 128.0.0.0. A /1 block holds 2,147,483,648 addresses with 2,147,483,646 usable hosts — ranges, wildcard, binary math and FAQs.

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# /1 Subnet Mask ReferenceIPv4

Halve the IPv4 universe once and you get a /1: 2,147,483,648 addresses that share a single leading binary one. Almost nobody configures one, yet the prefix is the clearest demonstration of how CIDR scales.

The /1 subnet mask is 128.0.0.0, equivalent to 127.255.255.255 as a wildcard. A /1 block contains 2,147,483,648 IP addresses (2,147,483,646 usable hosts).

## /1 facts at a glance

- **Subnet mask:** 128.0.0.0
- **Wildcard mask:** 127.255.255.255
- **Binary mask:** 10000000.00000000.00000000.00000000
- **Total addresses:** 2,147,483,648
- **Usable hosts:** 2,147,483,646 (excludes network & broadcast)
- **Block size:** 2,147,483,648 addresses (231)
- **Typical role:** Aggregation and supernetting

## Example /1 networks

A /1 spans a large share of the address space, so one aligned example says it all.

| Network | Usable range | Usable hosts |
| --- | --- | --- |
| 128.0.0.0/1 | 128.0.0.1 – 255.255.255.254 | 2,147,483,646 |

## How to calculate a /1 mask

A /1 mask sets the first 1 bits of a 32-bit number to 1 and leaves the remaining 31 bits as 0. In binary that reads `10000000.00000000.00000000.00000000`, which converts to dotted decimal `128.0.0.0`. Each block covers a contiguous run of 231 = 2,147,483,648 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving `127.255.255.255`.

## Where /1 is used

- Binary subnetting instruction: 0.0.0.0/1 and 128.0.0.0/1 are the two halves of IPv4.
- Supernetting and aggregation exercises, including certification exam questions.
- Occasional firewall policy grouping when an operator wants one rule per half of the space.

The mask 128.0.0.0 surprises people because it is simply the value 128 written in dotted decimal; its wildcard partner is 127.255.255.255.

## Frequently asked questions

**How many addresses are in a /1?**

2,147,483,648 in total, half of the IPv4 space. After the conventional subtraction of network and broadcast placeholders, 2,147,483,646 remain.

**What is the subnet mask for a /1?**

128.0.0.0, with wildcard 127.255.255.255. A single leading one-bit fixes the top of the address and leaves 31 bits for hosts.

**Would anyone ever deploy a /1?**

Effectively no. No organization approaches billions of connected hosts, and registries allocate far smaller blocks. The prefix earns its keep in teaching and aggregation theory.

## Open this block in a calculator

[Open the /1 block in the subnet calculator](https://itinsighthub.com/subnet-calculator/?q=128.0.0.0%2F1)

[Get the /1 wildcard mask →](https://itinsighthub.com/wildcard-mask-calculator/?q=%2F1)

### Related prefixes

[/0](https://itinsighthub.com/subnet/0/)[/2](https://itinsighthub.com/subnet/2/)[/24](https://itinsighthub.com/subnet/24/)[/26](https://itinsighthub.com/subnet/26/)[/28](https://itinsighthub.com/subnet/28/)[/30](https://itinsighthub.com/subnet/30/)

← /0 subnet mask
/2 subnet mask →

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