More detailsNeighboring subnets, wildcard mask, hex, reverse DNS, and binary breakdown
Neighboring /21 subnets
- Wildcard mask
- Used in Cisco ACLs and OSPF
- CIDR notation
- IP in hexadecimal
- IP as 32-bit integer
- Reverse DNS (PTR)
- Host bits / subnet bits
Binary breakdown
IP address
11000000.10101000.00000000.00000000Subnet mask
11111111.11111111.11111000.00000000Network
11000000.10101000.00000000.00000000Broadcast
11000000.10101000.00000111.11111111Quick facts for /21
- Subnet mask: 255.255.248.0
- Wildcard: 0.0.7.255
- Usable hosts: 2,046
- Total addresses: 2,048
/21 reference guideBit split, overview, sizing tables, notable networks, and FAQ
Binary bit split
11111111·11111111·11111000·00000000
Network bits (21)Host bits (11)
- Network bits
- 21
- Host bits
- 11
- Subnet mask
- 255.255.248.0
- Wildcard mask
- 0.0.7.255
- Total addresses
- 2,048
- Usable hosts
- 2,046
- Hosts formula
- 2^11 − 2
- /24 relationship
- 8 × /24 networks
Overview
A /21 block contains 2,048 addresses with 2,046 usable. Used for medium-sized segments in enterprise environments and cloud VPCs.
Common use cases
- Enterprise building or floor segment
- Cloud VPC subnet with room to grow
- ISP business customer allocation
Subnet sizing reference
Frequently asked questions
Common questions about /21 networks, subnet masks, and use cases.