255.255.255.128 Subnet Mask
255.255.255.128 is the subnet mask for a /25 network — 126 usable hosts out of 128 addresses.
CIDR Prefix
/25
Wildcard Mask
0.0.0.127
Total Addresses
128
Usable Hosts
126
Network Bits
25
Host Bits
7
255.255.255.128 in Binary
Each octet is eight bits. The highlighted bits are the 25 network bits; the rest are the 7 host bits that vary within the subnet.
255
255
255
128
Networks Using 255.255.255.128
| Network | Host Range | Broadcast |
|---|---|---|
| 10.0.0.0/25 | 10.0.0.1 – 10.0.0.126 | 10.0.0.127 |
| 172.16.0.0/25 | 172.16.0.1 – 172.16.0.126 | 172.16.0.127 |
| 192.168.1.0/25 | 192.168.1.1 – 192.168.1.126 | 192.168.1.127 |
Neighbouring Masks
Frequently Asked Questions
What CIDR prefix is 255.255.255.128?
255.255.255.128 is /25 in CIDR notation. The mask sets the first 25 of the 32 bits in an IPv4 address to 1, marking them as the network portion, and leaves the remaining 7 bits as 0 for hosts.
How many hosts does 255.255.255.128 allow?
A network with mask 255.255.255.128 holds 128 addresses, of which 126 are usable by hosts — the network address and the broadcast address are reserved.
What is the wildcard mask for 255.255.255.128?
The wildcard mask for 255.255.255.128 is 0.0.0.127. 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 /25 prefix notation that 255.255.255.128 is equivalent to, and requires masks to be contiguous — which is why exactly 33 of the 4.3 billion possible values are valid.