What a /20 subnet is: sixteen /24s
A /20 holds 4,096 addresses, 4,094 usable, and contains exactly sixteen /24 networks. It is a common per-environment allocation in cloud accounts.
At a glance
- Subnet mask
- 255.255.240.0
- Wildcard mask
- 0.0.15.255
- Total addresses
- 4,096
- Usable hosts
- 4,094
- Contains
- 16 networks of /24
- Host bits
- 12 of 32
In detail
The mask is 255.255.240.0. The third octet advances in steps of 16, so 10.1.0.0/20, 10.1.16.0/20 and 10.1.32.0/20 are consecutive blocks.
Big enough that a team will not come back for more addresses next quarter, small enough that a routing table full of them stays readable. Cloud providers often use a /20 as the smallest sensible virtual network.
Allocate on /20 boundaries even when the immediate need is one /24. Handing out the /24 from the middle of the space is what makes the next allocation impossible to summarise.
Examples
10.1.16.0/20 10.1.16.0 to 10.1.31.255, 4094 usable hosts
contains:
10.1.16.0/24, 10.1.17.0/24, ... 10.1.31.0/24 (16 networks)Things that catch people out
- The valid third octets for a /20 are multiples of 16, so 10.1.20.0/20 is not a network address even though 10.1.20.0/24 is.
- Two /20s that are adjacent but not aligned on a /19 boundary cannot be summarised into one route.
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 /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 /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 /16 subnet is: a full class B block
A /16 uses the mask 255.255.0.0 and gives 65,534 usable addresses. What it is for and where it goes wrong.
What a /21 subnet is: eight /24s
A /21 uses the mask 255.255.248.0 and gives 2,046 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.
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