/21 Subnet: Mask 255.255.248.0, 2,046 Usable Hosts

A /21 subnet has a subnet mask of 255.255.248.0 and contains 2,048 total IP addresses, of which 2,046 are usable host addresses (the network and broadcast addresses are reserved). A /21 contains 8 /24 networks.

Usable hosts in a /21 by platform

Cloud providers reserve more than the usual two addresses per subnet, so the same /21 gives you fewer usable IPs in the cloud than on a standard LAN.

Good to know

A /21 subnet has a 255.255.248.0 mask and 2,048 total addresses (2,046 usable). Each step from /21 to /22 halves the block; each step back to /20 doubles it.

How a /21 is calculated

• The /21 means the first 21 bits are the network portion, leaving 11 host bits.
• Total addresses = 2^(32 - 21) = 2^11 = 2,048.
• Usable hosts (standard) = 2,048 - 2 (network + broadcast) = 2,046.
• Subnet mask = 255.255.248.0; wildcard mask = 0.0.7.255.