What a /18 subnet is: a quarter of a /16
A /18 holds 16,384 addresses, 16,382 usable, and divides a /16 into exactly four equal pieces.
At a glance
- Subnet mask
- 255.255.192.0
- Wildcard mask
- 0.0.63.255
- Total addresses
- 16,384
- Usable hosts
- 16,382
- Contains
- 64 networks of /24
- Host bits
- 14 of 32
In detail
The mask is 255.255.192.0. The third octet moves in steps of 64, so the four networks inside a /16 start at .0.0, .64.0, .128.0 and .192.0.
A sensible unit when you want to split a site allocation between four availability zones, environments or floors, and still leave each one enough room to grow into /24s.
Write down the plan before you assign anything. A /18 split that starts at an arbitrary third octet cannot be summarised later, and the whole point of using regular boundaries is that it can.
Examples
10.20.0.0/16 split into four /18s:
10.20.0.0/18 10.20.0.0 to 10.20.63.255
10.20.64.0/18 10.20.64.0 to 10.20.127.255
10.20.128.0/18 10.20.128.0 to 10.20.191.255
10.20.192.0/18 10.20.192.0 to 10.20.255.255Things that catch people out
- 10.20.50.0/18 is not a network address. Valid third octets for a /18 are multiples of 64.
- A /18 spans 64 values of the third octet, so a firewall rule written per third octet needs 64 lines unless it is written as a prefix.
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 /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 /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 /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 /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 /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.
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