/20 subnet mask (255.255.240.0)

A /20 network has 12 host bits, 4,096 addresses and 4,094 usable hosts. It is 16 /24 networks.

/20 at a glance

Netmask: 255.255.240.0
Wildcard mask: 0.0.15.255
Hex mask: 0xFFFFF000
Binary mask: 11111111.11111111.11110000.00000000
Host bits: 12
Total addresses: 4,096
Usable hosts: 4,094 (minus network and broadcast)
Compared with a /24: 16 /24 networks

Calculate a /20 in the subnet calculator

Typical uses of a /20

A /20 (4,096 addresses) is what AWS uses for each subnet in the default VPC (172.31.0.0/16 split into /20s per Availability Zone). Docker also hands out /20 networks from 192.168.0.0/16 once its /16 pools run out.

Well-known /20 blocks

172.31.0.0/20: First subnet of the AWS default VPC (172.31.0.0/16 is split into one /20 per Availability Zone)

Example /20 ranges

Network	Network address	First usable	Last usable	Broadcast	Next /20
10.0.0.0/20	10.0.0.0	10.0.0.1	10.0.15.254	10.0.15.255	10.0.16.0/20	calculate
172.16.0.0/20	172.16.0.0	172.16.0.1	172.16.15.254	172.16.15.255	172.16.16.0/20	calculate
192.168.0.0/20	192.168.0.0	192.168.0.1	192.168.15.254	192.168.15.255	192.168.16.0/20	calculate

The /20 boundary falls in the third octet, so /20 networks start every 16 in that octet (16 per /16): 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240.

How many /20s fit

In a	Number of /20 subnets
/8	4,096
/16	16

What a /20 contains

Smaller block	How many
/24	16
/26	64
/28	256
/29	512
/30	1,024
/32	4,096

Splitting a /20

Into	Subnets	Usable hosts each
/21	2	2,046
/22	4	1,022
/23	8	510
/24	16	254

/20 in AWS, Azure and Google Cloud

Platform	Allowed subnet sizes	/20 allowed?	Usable after reservation
AWS VPC	/16 to /28	Yes	4,091 (5 reserved)
Azure VNet	/2 to /29	Yes	4,091 (5 reserved)
Google Cloud VPC	/4 to /29	Yes	4,092 (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.