The /20 prefix: 4,096 addresses for serious cloud networks
A /20 sits between the octet boundaries: 20 network bits, 12 host bits, 4,096 addresses under the mask 255.255.240.0. In dotted-quad terms the third octet moves in steps of 16 — 10.0.0.0/20 spans 10.0.0.0 through 10.0.15.255.
It is the size cloud architects pick almost by reflex. A /20 is sixteen /24s, enough room to spread tiers, availability zones, and years of growth without hoarding address space you cannot justify.
| Prefix length | /20 |
| Subnet mask | 255.255.240.0 |
| Wildcard mask | 0.0.15.255 |
| Total addresses | 4,096 |
| Usable hosts | 4,094 |
The cloud sweet spot
AWS guidance is to size VPC subnets generously, and a /20 per VPC — or per large segment — is the common answer: 4,094 usable addresses survives autoscaling spikes, Lambda functions grabbing ENIs, and load balancers multiplying across zones. Azure's advice rhymes: plan bigger than feels necessary, because resizing a VPC CIDR later is surgery.
Reading the non-octet boundary
Because /20 does not land on an octet, people misread its boundaries constantly. The mask's third octet is 240, meaning the top four bits of that octet belong to the network. Valid /20 network addresses have third octets of 0, 16, 32, up to 240 — sixteen blocks per /16. Memorize the multiples of 16 and half of your subnetting mistakes disappear.
CIDR Calculator
Run the math on 10.0.0.0/20 — or any other block
Frequently asked questions
How many IP addresses are in a /20?
4,096 in total (2^12 host bits), of which 4,094 are usable for hosts.
What is the subnet mask of a /20?
255.255.240.0. The 240 means the top four bits of the third octet are network bits, so blocks advance in steps of 16 in that octet.
How many /24s fit in a /20?
Sixteen. Each step down from /24 doubles the block: /23 is two /24s, /22 is four, /21 is eight, /20 is sixteen.
Is a /20 a good VPC size?
Yes — it is the most common production VPC size. Roomy enough for multi-AZ layouts with per-tier subnets, small enough that several /20s still fit inside a private /16.