128.0.0.0 Subnet Mask
128.0.0.0 is the subnet mask for a /1 network — 2,147,483,646 usable hosts out of 2,147,483,648 addresses.
CIDR Prefix
/1
Wildcard Mask
127.255.255.255
Total Addresses
2,147,483,648
Usable Hosts
2,147,483,646
Network Bits
1
Host Bits
31
128.0.0.0 in Binary
Each octet is eight bits. The highlighted bits are the 1 network bits; the rest are the 31 host bits that vary within the subnet.
128
0
0
0
Networks Using 128.0.0.0
| Network | Host Range | Broadcast |
|---|---|---|
| 0.0.0.0/1 | 0.0.0.1 – 127.255.255.254 | 127.255.255.255 |
| 128.0.0.0/1 | 128.0.0.1 – 255.255.255.254 | 255.255.255.255 |
| 128.0.0.0/1 | 128.0.0.1 – 255.255.255.254 | 255.255.255.255 |
Neighbouring Masks
Frequently Asked Questions
What CIDR prefix is 128.0.0.0?
128.0.0.0 is /1 in CIDR notation. The mask sets the first 1 of the 32 bits in an IPv4 address to 1, marking them as the network portion, and leaves the remaining 31 bits as 0 for hosts.
How many hosts does 128.0.0.0 allow?
A network with mask 128.0.0.0 holds 2,147,483,648 addresses, of which 2,147,483,646 are usable by hosts — the network address and the broadcast address are reserved.
What is the wildcard mask for 128.0.0.0?
The wildcard mask for 128.0.0.0 is 127.255.255.255. It is the bitwise inverse: every 1 in the subnet mask becomes a 0 and every 0 becomes a 1. Cisco access lists and OSPF network statements take the wildcard form rather than the subnet mask.
Standards
- RFC 950 — Internet Standard Subnetting Procedure
Defines the dotted-decimal subnet mask itself, and the rule that the network and broadcast addresses are not assignable to hosts.
- RFC 4632 — Classless Inter-Domain Routing (CIDR)
Establishes the /1 prefix notation that 128.0.0.0 is equivalent to, and requires masks to be contiguous — which is why exactly 33 of the 4.3 billion possible values are valid.