What a /21 subnet is: eight /24s
A /21 holds 2,048 addresses, 2,046 usable, and contains eight /24 networks. It sits between a /20 and a /22 when neither is the right size.
At a glance
- Subnet mask
- 255.255.248.0
- Wildcard mask
- 0.0.7.255
- Total addresses
- 2,048
- Usable hosts
- 2,046
- Contains
- 8 networks of /24
- Host bits
- 11 of 32
In detail
The mask is 255.255.248.0 and the third octet steps by 8. It is the smallest block that comfortably holds a thousand or so devices without pushing you up to a /20.
Reasonable for a floor of a large office, or a wireless network where devices outnumber people three to one and lease churn keeps the pool busy.
If you are sizing for wireless, count devices and not users, then double it. Address exhaustion on a guest network shows up as intermittent connection failures rather than as an obvious error.
Examples
10.30.8.0/21 10.30.8.0 to 10.30.15.255, 2046 usable hosts
adjacent blocks:
10.30.0.0/21 10.30.16.0/21 10.30.24.0/21Things that catch people out
- Valid third octets for a /21 are multiples of 8. 10.30.10.0/21 is not a network address.
- A large flat /21 is still one broadcast domain. Two thousand wireless clients on one segment will spend a noticeable amount of airtime on broadcast traffic.
Try it: IP Subnet Calculator
Enter any IPv4 address with a prefix or a dotted mask and get the network address, broadcast address, wildcard mask, usable host range, address class, and the hex and binary forms.
IP Subnet Calculator
LOCAL · Browser-OnlyCalculate IP subnet details, network ranges, and CIDR notation
192.168.1.0192.168.1.255255.255.255.00.0.0.255192.168.1.0/24192.168.1.1 - 192.168.1.254192.168.1.1192.168.1.254256254C0xC0A8016411000000.10101000.00000001.01100100Split into two /25 subnets
Dividing 192.168.1.0/24 one prefix deeper.
Common Subnet Masks
Related IPv4 subnets
All ipv4 subnets →What a /22 subnet is: four /24s
A /22 uses the mask 255.255.252.0 and gives 1,022 usable addresses. What it is for and where it goes wrong.
What a /20 subnet is: sixteen /24s
A /20 uses the mask 255.255.240.0 and gives 4,094 usable addresses. What it is for and where it goes wrong.
What a /24 subnet is: the classic LAN subnet
A /24 uses the mask 255.255.255.0 and gives 254 usable addresses. What it is for and where it goes wrong.
What a /23 subnet is: two /24s
A /23 uses the mask 255.255.254.0 and gives 510 usable addresses. What it is for and where it goes wrong.
What a /25 subnet is: half a /24
A /25 uses the mask 255.255.255.128 and gives 126 usable addresses. What it is for and where it goes wrong.
What a /26 subnet is: a quarter of a /24
A /26 uses the mask 255.255.255.192 and gives 62 usable addresses. What it is for and where it goes wrong.
Tools for this
- IP Subnet Calculator Free online IP subnet calculator - calculate IP subnet details, CIDR notation, and network address ranges
- IP Address Lookup Free online IP address lookup - find your public IP address and basic network information
- Number Base Converter Free online number base converter - convert numbers between decimal, binary, octal, and hexadecimal