Toolman

Subnet mask 255.255.255.128

/25

255.255.255.128 · 126 usable hosts

CIDR prefix/25
Subnet mask255.255.255.128
Wildcard mask0.0.0.127
Binary11111111.11111111.11111111.10000000
Total addresses128
Usable hosts126

How many hosts it holds

128 addresses, of which 126 can be assigned to hosts — the first is the network address and the last is the broadcast, and neither goes on an interface.

Reading the mask

A subnet mask is 25 one-bits followed by 7 zero-bits, written as four decimal octets. The ones mark the part of the address that identifies the network and the zeros mark the part that identifies the host, which is why the mask is always a solid run of ones — a mask with a gap in it would split the host part in two and is not valid.

11111111.11111111.11111111.10000000
255.255.255.128

The prefix length is simply the count of those ones, so 255.255.255.128 is /25 and nothing has to be calculated to convert between them.

The wildcard, and where it turns up

Cisco access lists and OSPF network statements take a wildcard mask rather than a subnet mask — the same value with every bit inverted, 0.0.0.127 here. It trips people up because the two look alike and mean opposite things: in a subnet mask a one means "this bit is the network", and in a wildcard a one means "ignore this bit".

Frequently asked questions

What CIDR prefix is 255.255.255.128?

255.255.255.128 is /25. The mask has 25 bits set, and the prefix length is that count.

How many hosts does 255.255.255.128 allow?

128 addresses in total and 126 usable, after the network and broadcast addresses are taken out.

What is the wildcard mask for 255.255.255.128?

0.0.0.127 — the mask with every bit flipped. Cisco access lists and OSPF network statements take the wildcard rather than the mask, which is the usual reason to want it.

Other masks

/25 in full · Every subnet mask · CIDR reference