255.128.0.0 Subnet Mask
255.128.0.0 is the subnet mask for a /9 network — 8,388,606 usable hosts out of 8,388,608 addresses.
CIDR Prefix
/9
Wildcard Mask
0.127.255.255
Total Addresses
8,388,608
Usable Hosts
8,388,606
Network Bits
9
Host Bits
23
255.128.0.0 in Binary
Each octet is eight bits. The highlighted bits are the 9 network bits; the rest are the 23 host bits that vary within the subnet.
255
128
0
0
Networks Using 255.128.0.0
| Network | Host Range | Broadcast |
|---|---|---|
| 10.0.0.0/9 | 10.0.0.1 – 10.127.255.254 | 10.127.255.255 |
| 172.0.0.0/9 | 172.0.0.1 – 172.127.255.254 | 172.127.255.255 |
| 192.128.0.0/9 | 192.128.0.1 – 192.255.255.254 | 192.255.255.255 |
Neighbouring Masks
Frequently Asked Questions
What CIDR prefix is 255.128.0.0?
255.128.0.0 is /9 in CIDR notation. The mask sets the first 9 of the 32 bits in an IPv4 address to 1, marking them as the network portion, and leaves the remaining 23 bits as 0 for hosts.
How many hosts does 255.128.0.0 allow?
A network with mask 255.128.0.0 holds 8,388,608 addresses, of which 8,388,606 are usable by hosts — the network address and the broadcast address are reserved.
What is the wildcard mask for 255.128.0.0?
The wildcard mask for 255.128.0.0 is 0.127.255.255. 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 /9 prefix notation that 255.128.0.0 is equivalent to, and requires masks to be contiguous — which is why exactly 33 of the 4.3 billion possible values are valid.