/21 Subnet Mask (255.255.248.0)
| Prefix | /21 |
|---|---|
| Subnet mask | 255.255.248.0 |
| Wildcard mask | 0.0.7.255 |
| Binary mask | 11111111.11111111.11111000.00000000 |
| Total addresses | 2,048 |
| Usable hosts | 2,046 |
| Host bits | 11 |
| Subnets of /21 in a /16 | 32 |
e.g. 10.0.0.5/22 or 10.0.0.5 255.255.252.0
| Network address | 172.16.0.0 | |
|---|---|---|
| Broadcast address | 172.16.7.255 | |
| First usable host | 172.16.0.1 | |
| Last usable host | 172.16.7.254 | |
| Total addresses | 2,048 | |
| Usable hosts | 2,046 | |
| Subnet mask | 255.255.248.0 | |
| Wildcard mask | 0.0.7.255 | |
| CIDR notation | 172.16.0.0/21 | |
| IP class | Class B (legacy) | |
| Address type | Private (RFC 1918) | |
| IP address (binary) | 10101100.00010000.00000000.00000000 | |
| Subnet mask (binary) | 11111111.11111111.11111000.00000000 |
■ network bits ■ host bits
| Requested hosts | CIDR | First host | Last host | Broadcast | Usable hosts |
|---|
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What is a /21 subnet?
A /21 subnet uses the first 21 bits of an IPv4 address for the network and leaves 11 bits for hosts. Its subnet mask is 255.255.248.0, and it contains 2¹¹ = 2,048 addresses, of which 2,046 can be assigned to devices. Typical use: Eight /24s (2,046 usable hosts); used for large flat networks, campus Wi-Fi or cloud subnets that need room to grow. Use the subnet calculator above to check any /21 network.
/21 subnet example
For 172.16.0.0/21: network address 172.16.0.0, usable hosts 172.16.0.1 – 172.16.7.254, broadcast address 172.16.7.255, wildcard mask 0.0.7.255.
/21 networks in 172.16.0.0/16
A /16 holds 32 /21 networks:
- 172.16.0.0/21
- 172.16.8.0/21
- 172.16.16.0/21
- 172.16.24.0/21
- 172.16.32.0/21
- 172.16.40.0/21
- 172.16.48.0/21
- 172.16.56.0/21
- 172.16.64.0/21
- 172.16.72.0/21
- 172.16.80.0/21
- 172.16.88.0/21
- 172.16.96.0/21
- 172.16.104.0/21
- 172.16.112.0/21
- 172.16.120.0/21
- 172.16.248.0/21
Frequently asked questions
How many usable IPs are in a /21?
2,046. The block has 2,048 addresses; the first is the network address and the last is the broadcast address.
What is the subnet mask for /21?
255.255.248.0, or 11111111.11111111.11111000.00000000 in binary.
How many /21 subnets are in a /16?
32 (2⁵), because a /21 uses 5 more network bits than a /16.
What is the difference between /21 and /20?
A /20 (255.255.240.0) is twice as large: 4,096 addresses and 4,094 usable hosts, spanning two consecutive /21 blocks such as 172.16.0.0–172.16.15.255.