An administrator needs to configure an IoT device with a /21 subnet mask, but the...

A /21 subnet mask means 21 bits are network bits:

8 + 8 + 5 = 21 → leaves 11 bits for hosts.

Subnet mask in binary: 11111111.11111111.11111000.00000000

Converted to decimal: 255.255.248.0

Therefore:

A. /19 = 255.255.224.0

B. /20 = 255.255.240.0

✅ C. /21 = 255.255.248.0 ← correct

D. /22 = 255.255.252.0

Answer correction: The correct option is C. 255.255.248.0, not B.

References (CompTIA Network+ N10-009):

Domain: Networking Concepts — Subnetting, CIDR notation, dotted decimal masks.