The /16 prefix: 65,536 addresses, the classic Class B
A /16 fixes the first two octets and leaves 16 host bits: 65,536 addresses under the mask 255.255.0.0. In classful terms this was a Class B network — the size universities and mid-to-large companies received before CIDR, big enough to be comfortable and small enough that there were 16,384 of them to hand out.
Today the /16 matters most as a ceiling and a default. It is the largest CIDR block AWS accepts for a VPC, it is the size of the 192.168.0.0/16 private range most home gear lives in, and it is the block a network team reaches for when planning stops fitting inside individual /24s.
| Prefix length | /16 |
| Subnet mask | 255.255.0.0 |
| Wildcard mask | 0.0.255.255 |
| Total addresses | 65,536 |
| Usable hosts | 65,534 |
Cloud VPCs and the /16 ceiling
AWS caps a VPC's CIDR at /16: you can go smaller, down to /28, but never larger. Azure VNets play by similar rules. So when you see 10.0.0.0/16 as a VPC's range, that is the provider saying here is your entire private universe — subnet it wisely. A common layout carves the /16 into /20s per availability zone and /24s per tier.
Splitting and aggregating
One /16 equals 256 /24s, which makes it a natural aggregation boundary: an ISP or enterprise can summarize a whole site's subnets into a single /16 route. Run the knife the other way and the same /16 yields 64 /22s or 16 /20s — clean powers of two all the way down, since 16 bits is exactly half the address.
CIDR Calculator
Run the math on 10.0.0.0/16 — or any other block
Frequently asked questions
How many IP addresses are in a /16?
65,536 in total (2^16), with 65,534 usable for hosts once the network and broadcast addresses are removed.
What is the subnet mask of a /16?
255.255.0.0. The first two octets are network bits; the last two are entirely yours.
Is a /16 the same as a Class B network?
Yes — Class B was the pre-CIDR label for a block with 16 host bits. The classes died with CIDR in 1993, but the /16 kept the role and the nickname.
Why do so many private networks use a /16?
Because two of the three RFC 1918 ranges are that shape or larger, and a /16 is roomy without being unmanageable: 256 subnets of /24 each, enough for any single site or VPC.