Toolman

Subnet mask 255.255.0.0

/16

255.255.0.0 · 65,534 usable hosts

CIDR prefix/16
Subnet mask255.255.0.0
Wildcard mask0.0.255.255
Binary11111111.11111111.00000000.00000000
Total addresses65,536
Usable hosts65,534

How many hosts it holds

65,536 addresses, of which 65,534 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 16 one-bits followed by 16 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.

11111111.11111111.00000000.00000000
255.255.0.0

The prefix length is simply the count of those ones, so 255.255.0.0 is /16 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, 0.0.255.255 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 255.255.0.0?

255.255.0.0 is /16. The mask has 16 bits set, and the prefix length is that count.

How many hosts does 255.255.0.0 allow?

65,536 addresses in total and 65,534 usable, after the network and broadcast addresses are taken out.

What is the wildcard mask for 255.255.0.0?

0.0.255.255 — 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

/16 in full · Every subnet mask · CIDR reference