/16 Subnet Mask ReferenceIPv4
The classic campus block: <code>x.x.0.0/16</code> packs 65,536 addresses, or 256 clean /24s, behind the memorably symmetric mask <code>255.255.0.0</code>. From home routers to AWS VPCs, the /16 is where most network designs begin.
/16 facts at a glance
- Subnet mask
- 255.255.0.0
- Wildcard mask
- 0.0.255.255
- Binary mask
- 11111111.11111111.00000000.00000000
- Total addresses
- 65,536
- Usable hosts
- 65,534 (excludes network & broadcast)
- Block size
- 65,536 addresses (216)
- Typical role
- Legacy Class B equivalent
Example /16 networks
Three consecutive /16 blocks, starting at the /16 boundary that contains 198.51.100.0:
| Network | Usable range | Usable hosts |
|---|---|---|
| 198.51.0.0/16 | 198.51.0.1 – 198.51.255.254 | 65,534 |
| 198.52.0.0/16 | 198.52.0.1 – 198.52.255.254 | 65,534 |
| 198.53.0.0/16 | 198.53.0.1 – 198.53.255.254 | 65,534 |
How to calculate a /16 mask
A /16 mask sets the first 16 bits of a 32-bit number to 1 and leaves the remaining 16 bits as 0. In binary that reads 11111111.11111111.00000000.00000000, which converts to dotted decimal 255.255.0.0. Each block covers a contiguous run of 216 = 65,536 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving 0.0.255.255.
Where /16 is used
- Consumer networking: home routers live inside RFC 1918's <code>192.168.0.0/16</code>, typically serving a /24 slice such as 192.168.1.0/24.
- Cloud ceilings: AWS caps a VPC's primary CIDR at /16, and its default VPC is famously <code>172.31.0.0/16</code>.
- Campus cores that advertise one /16 summary at the border and keep interior routing tables short.
- Route-summarization teaching, because 256 /24s collapsing into one line demonstrates the power of CIDR instantly.
Usable hosts come to 65,534: the lowest address names the network and the highest is the broadcast.
Clean subdivisions make planning easy: two /17s, four /18s, or 512 /25s all tile a /16 perfectly.
Frequently asked questions
How many usable hosts are in a /16?
65,534 of 65,536 total addresses. The two reserved slots are the network identifier and the broadcast address.
How many /24s are in a /16?
256. The third octet counts straight from 0 to 255, giving you 256 individually routable LANs inside one summary.
What is 192.168.0.0/16?
The smallest RFC 1918 private range. Everything from 192.168.0.0 through 192.168.255.255 is reusable behind NAT and unroutable publicly, which is why nearly every home router uses it.
Is a /16 too big for one flat LAN?
As a single broadcast domain, yes: tens of thousands of ARP-speaking hosts create needless churn. Keep the /16 as your allocation and carve it into /22 to /24 segments per switch or VLAN.
Open this block in a calculator
Open the /16 block in the subnet calculator