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/21 Subnet Mask: 255.255.248.0

A /21 prefix fixes the first 21 bits of the address as the network, leaving 11 bits for hosts. That gives 2,048 addresses, of which 2,046 can be assigned to hosts.

CIDR notation/21
Subnet mask255.255.248.0
Wildcard mask0.0.7.255
Binary mask11111111.11111111.11111000.00000000
Hex mask0xFFFFF800
Total addresses2,048
Usable hosts2,046

Example: 172.16.0.0/21

  • Network address: 172.16.0.0
  • First usable host: 172.16.0.1
  • Last usable host: 172.16.7.254
  • Broadcast address: 172.16.7.255

When to use a /21 subnet

Larger office or campus segments, and cloud subnets that need hundreds to thousands of addresses.

  • 8,192 /21 subnets fit in a /8.
  • 32 /21 subnets fit in a /16.

Frequently Asked Questions

What is the subnet mask for /21?

The subnet mask for /21 is 255.255.248.0, and its wildcard (inverse) mask is 0.0.7.255.

How many hosts are in a /21?

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