255255255128

255255255128, typically written as 255.255.255.128, is an IPv4 subnet mask used to subdivide a larger network into smaller subnets. It corresponds to the binary mask 11111111.11111111.11111111.10000000 and to the CIDR prefix /25, meaning the first 25 bits are fixed as the network portion.

With a /25 mask, a single /24 network is divided into two subnets: x.x.x.0/25 (addresses .0–.127) and x.x.x.128/25 (addresses .128–.255). Each subnet contains 128 total addresses, of which 126 are typically usable for hosts; the remaining addresses serve as the network address (.0 or .128) and the broadcast address (.127 or .255) for that subnet.

In practical use, 255.255.255.128 is employed in routing and addressing to control subnet sizes within IPv4 networks. It is common in small or segmented networks and is frequently configured on residential or enterprise routers to create two equal subnets from a larger /24 block. The mask is specific to IPv4; IPv6 uses a different addressing scheme and prefixing method. In administration and documentation, the dotted-decimal form is the standard representation for human configuration, while the underlying meaning is the fixed 25-bit network portion and a 7-bit host portion.