/12 subnet mask (255.240.0.0)

A /12 network has 20 host bits, 1,048,576 addresses and 1,048,574 usable hosts. It is 4,096 /24 networks.

/12 at a glance

Netmask: 255.240.0.0
Wildcard mask: 0.15.255.255
Hex mask: 0xFFF00000
Binary mask: 11111111.11110000.00000000.00000000
Host bits: 20
Total addresses: 1,048,576
Usable hosts: 1,048,574 (minus network and broadcast)
Compared with a /24: 4,096 /24 networks

Calculate a /12 in the subnet calculator

Typical uses of a /12

A /12 is the size of 172.16.0.0/12 (RFC 1918), which runs from 172.16.0.0 to 172.31.255.255. Kubernetes clusters built with kubeadm use 10.96.0.0/12 for Service IPs by default. Docker carves its default bridge networks out of this range too.

Well-known /12 blocks

172.16.0.0/12: Private network (RFC 1918)
10.96.0.0/12: kubeadm default Kubernetes service CIDR

Example /12 ranges

Network	Network address	First usable	Last usable	Broadcast	Next /12
10.0.0.0/12	10.0.0.0	10.0.0.1	10.15.255.254	10.15.255.255	10.16.0.0/12	calculate
172.16.0.0/12	172.16.0.0	172.16.0.1	172.31.255.254	172.31.255.255	172.32.0.0/12	calculate

The /12 boundary falls in the second octet, so /12 networks start every 16 in that octet (16 per /8): 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240.

How many /12s fit

In a	Number of /12 subnets
/8	16

What a /12 contains

Smaller block	How many
/16	16
/24	4,096
/26	16,384
/28	65,536
/29	131,072
/30	262,144
/32	1,048,576

Splitting a /12

Into	Subnets	Usable hosts each
/13	2	524,286
/14	4	262,142
/15	8	131,070
/16	16	65,534

/12 in AWS, Azure and Google Cloud

Platform	Allowed subnet sizes	/12 allowed?	Usable after reservation
AWS VPC	/16 to /28	No	-
Azure VNet	/2 to /29	Yes	1,048,571 (5 reserved)
Google Cloud VPC	/4 to /29	Yes	1,048,572 (4 reserved)

Cloud providers reserve addresses in every subnet (first four and the last address (network, VPC router, DNS, future use, broadcast) on AWS), so the usable count there differs from the classic 2^n - 2.