/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.
/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: <code>0.0.0.0/1</code> and <code>128.0.0.0/1</code> 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 <code>128.0.0.0</code> surprises people because it is simply the value 128 written in dotted decimal; its wildcard partner is <code>127.255.255.255</code>.
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?
<code>128.0.0.0</code>, with wildcard <code>127.255.255.255</code>. 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