/21 subnet mask (255.255.248.0)

A /21 network has 11 host bits, 2,048 addresses and 2,046 usable hosts. It is 8 /24 networks.

/21 at a glance

Netmask: 255.255.248.0
Wildcard mask: 0.0.7.255
Hex mask: 0xFFFFF800
Binary mask: 11111111.11111111.11111000.00000000
Host bits: 11
Total addresses: 2,048
Usable hosts: 2,046 (minus network and broadcast)
Compared with a /24: 8 /24 networks

Calculate a /21 in the subnet calculator

Typical uses of a /21

A /21 is eight /24s, often used for a building, a campus Wi-Fi network or a mid-size VPC subnet that has to hold a few thousand hosts.

Example /21 ranges

Network	Network address	First usable	Last usable	Broadcast	Next /21
10.0.0.0/21	10.0.0.0	10.0.0.1	10.0.7.254	10.0.7.255	10.0.8.0/21	calculate
172.16.0.0/21	172.16.0.0	172.16.0.1	172.16.7.254	172.16.7.255	172.16.8.0/21	calculate
192.168.0.0/21	192.168.0.0	192.168.0.1	192.168.7.254	192.168.7.255	192.168.8.0/21	calculate

The /21 boundary falls in the third octet, so /21 networks start every 8 in that octet (32 per /16): 0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120, ....

How many /21s fit

In a	Number of /21 subnets
/8	8,192
/16	32
/20	2

What a /21 contains

Smaller block	How many
/24	8
/26	32
/28	128
/29	256
/30	512
/32	2,048

Splitting a /21

Into	Subnets	Usable hosts each
/22	2	1,022
/23	4	510
/24	8	254
/25	16	126

/21 in AWS, Azure and Google Cloud

Platform	Allowed subnet sizes	/21 allowed?	Usable after reservation
AWS VPC	/16 to /28	Yes	2,043 (5 reserved)
Azure VNet	/2 to /29	Yes	2,043 (5 reserved)
Google Cloud VPC	/4 to /29	Yes	2,044 (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.