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/21 Subnet Mask

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A /21 subnet mask is 255.255.248.0, with 2,048 addresses and 2,046 usable hosts.

CIDR prefix/21
Subnet mask255.255.248.0
Subnet mask (binary)11111111.11111111.11111000.00000000
Wildcard mask0.0.7.255
Network bits / host bits21 / 11
Total addresses2,048
Usable hosts2,046
/21 subnets in a /88,192
/21 subnets in a /1632

The mask ends 5 bits into octet 3, so blocks step in increments of 8 in that octet.

Try it with an address

Example /21 subnets in 192.168.0.0/16

Network First usable host Last usable host Broadcast
192.168.0.0/21 192.168.0.1 192.168.7.254 192.168.7.255
192.168.8.0/21 192.168.8.1 192.168.15.254 192.168.15.255
192.168.16.0/21 192.168.16.1 192.168.23.254 192.168.23.255
192.168.24.0/21 192.168.24.1 192.168.31.254 192.168.31.255
192.168.32.0/21 192.168.32.1 192.168.39.254 192.168.39.255
192.168.40.0/21 192.168.40.1 192.168.47.254 192.168.47.255
192.168.48.0/21 192.168.48.1 192.168.55.254 192.168.55.255
192.168.56.0/21 192.168.56.1 192.168.63.254 192.168.63.255

Frequently asked questions

What is the subnet mask for /21?

A /21 subnet mask is 255.255.248.0. In binary that is 11111111.11111111.11111000.00000000: 21 network bits followed by 11 host bits. The wildcard mask is 0.0.7.255.

How many usable hosts are in a /21?

A /21 holds 2,048 addresses in total. Subtracting the network and broadcast addresses leaves 2,046 usable hosts.

How many /21 subnets fit in a /16?

A /16 divides into 32 subnets of size /21, because each extra mask bit doubles the number of subnets.

Other prefix lengths

← /20 Full CIDR chart /22 →

Calculate this over HTTP: curl https://networkcalc.com/api/ip/192.168.1.0/21. See the Subnet Calculator API.