HTTPfy

/1 subnet mask (128.0.0.0)

A /1 network has 31 host bits, 2,147,483,648 addresses and 2,147,483,646 usable hosts. It is 8,388,608 /24 networks.

/1 at a glance

Netmask
128.0.0.0
Wildcard mask
127.255.255.255
Hex mask
0x80000000
Binary mask
10000000.00000000.00000000.00000000
Host bits
31
Total addresses
2,147,483,648
Usable hosts
2,147,483,646 (minus network and broadcast)
Compared with a /24
8,388,608 /24 networks
Calculate a /1 in the subnet calculator

Typical uses of a /1

Two /1s (0.0.0.0/1 and 128.0.0.0/1) together cover everything, and because each is more specific than 0.0.0.0/0 they win over the default route. VPN clients use this trick (OpenVPN def1, many WireGuard configs) to capture all traffic while leaving the original default route in place for the tunnel endpoint itself.

Well-known /1 blocks

  • 0.0.0.0/1 + 128.0.0.0/1: OpenVPN "redirect-gateway def1" pair that overrides the default route without deleting it

Example /1 ranges

NetworkNetwork addressFirst usableLast usableBroadcastNext /1
0.0.0.0/1 0.0.0.0 0.0.0.1 127.255.255.254 127.255.255.255 128.0.0.0/1 calculate

The /1 boundary falls in the first octet, so /1 networks start every 128 in that octet (2 per address space): 0, 128.

What a /1 contains

Smaller blockHow many
/1632,768
/248,388,608
/2633,554,432
/28134,217,728
/29268,435,456
/30536,870,912
/322,147,483,648

Splitting a /1

IntoSubnetsUsable hosts each
/221,073,741,822
/34536,870,910
/48268,435,454
/516134,217,726

/1 in AWS, Azure and Google Cloud

PlatformAllowed subnet sizes/1 allowed?Usable after reservation
AWS VPC/16 to /28 No -
Azure VNet/2 to /29 No -
Google Cloud VPC/4 to /29 No -

Cloud providers reserve addresses in every subnet (first four and the last address (network, VPC router, DNS, future use, broadcast) on AWS), so the usable count there differs from the classic 2^n - 2.