More detailsNeighboring subnets, wildcard mask, hex, reverse DNS, and binary breakdown
Neighboring /1 subnets
- Wildcard mask
- Used in Cisco ACLs and OSPF
- CIDR notation
- IP in hexadecimal
- IP as 32-bit integer
- Reverse DNS (PTR)
- Host bits / subnet bits
Binary breakdown
IP address
00000000.00000000.00000000.00000000Subnet mask
10000000.00000000.00000000.00000000Network
00000000.00000000.00000000.00000000Broadcast
01111111.11111111.11111111.11111111Quick facts for /1
- Subnet mask: 128.0.0.0
- Wildcard: 127.255.255.255
- Usable hosts: 2,147,483,646
- Total addresses: 2,147,483,648
/1 reference guideBit split, overview, sizing tables, notable networks, and FAQ
Binary bit split
10000000·00000000·00000000·00000000
Network bits (1)Host bits (31)
- Network bits
- 1
- Host bits
- 31
- Subnet mask
- 128.0.0.0
- Wildcard mask
- 127.255.255.255
- Total addresses
- 2,147,483,648
- Usable hosts
- 2,147,483,646
- Hosts formula
- 2^31 − 2
- /24 relationship
- 8,388,608 × /24 networks
Overview
A /1 block covers half of the IPv4 address space — 2,147,483,648 addresses. A pair of /1 routes is sometimes used to override the default route in VPN and BGP configurations.
Common use cases
- VPN split-tunneling with two /1 routes to capture all traffic
- Large-scale BGP route aggregation
- Half-internet routing in carrier networks
Subnet sizing reference
Notable /1 networks
0.0.0.0/1Lower half of IPv4 space128.0.0.0/1Upper half of IPv4 space
Frequently asked questions
Common questions about /1 networks, subnet masks, and use cases.