/21 subnet: mask 255.255.248.0, 2,048 addresses
Everything about a /21 network, with the CIDR calculator pre-filled — change the IP to calculate your own block.
Is an IP inside a CIDR block?
Yes — 10.0.42.7 is inside 10.0.0.0/16.
Do two CIDR blocks overlap?
Overlap — 10.0.128.0/20 sits inside 10.0.0.0/16 (10.0.128.0 – 10.0.143.255).
CIDR blocks never partly overlap: one is inside the other, or they don't touch at all. Overlapping VPCs can't be peered.
/21 at a glance
- Subnet mask
- 255.255.248.0
- Wildcard mask
- 0.0.7.255
- Mask in binary
- 11111111.11111111.11111000.00000000
- Host bits
- 11
- Total addresses
- 2,048
- Usable hosts
- 2,046 (network and broadcast address excluded)
- Usable in an AWS or Azure subnet
- 2,043 (5 reserved per subnet)
- Equivalent /24 networks
- 8
Eight /24s together — a medium office or a VPC split across a few availability zones.
Splitting a /21 into smaller subnets
Frequently asked
What is the subnet mask for /21?
255.255.248.0. The first 21 bits are network bits and the remaining 11 are host bits; the wildcard (inverse) mask is 0.0.7.255.
How many IP addresses are in a /21?
2,048 in total. 2,046 of them can be assigned to hosts, because the first is the network address and the last is the broadcast address. In an AWS VPC or Azure subnet, 2,043 are usable because 5 are reserved.
CIDR to subnet mask cheat sheet
| CIDR | Subnet mask | Addresses | Usable hosts |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
| /12 | 255.240.0.0 | 1,048,576 | 1,048,574 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
| /17 | 255.255.128.0 | 32,768 | 32,766 |
| /18 | 255.255.192.0 | 16,384 | 16,382 |
| /19 | 255.255.224.0 | 8,192 | 8,190 |
| /20 | 255.255.240.0 | 4,096 | 4,094 |
| /21 | 255.255.248.0 | 2,048 | 2,046 |
| /22 | 255.255.252.0 | 1,024 | 1,022 |
| /23 | 255.255.254.0 | 512 | 510 |
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
| /31 | 255.255.255.254 | 2 | 2 |
| /32 | 255.255.255.255 | 1 | 1 |
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