What a /23 subnet is: two /24s
A /23 holds 512 addresses, 510 usable, and joins two /24 networks into one. It is the smallest step up from a /24 that people actually take.
At a glance
- Subnet mask
- 255.255.254.0
- Wildcard mask
- 0.0.1.255
- Total addresses
- 512
- Usable hosts
- 510
- Contains
- 2 networks of /24
- Host bits
- 9 of 32
In detail
The mask is 255.255.254.0. The third octet must be even, and the range runs from that octet through the next one, so 10.0.6.0/23 covers 10.0.6.0 to 10.0.7.255.
The right move when a /24 is at 240 hosts and climbing. It doubles the space without needing three neighbouring blocks to be free the way a /22 does.
Check the third octet is even. Half of all attempted /23s fail because the network was numbered on an odd boundary and the calculator quietly shows the block starting one lower than expected.
Examples
10.0.6.0/24 + 10.0.7.0/24
= 10.0.6.0/23 10.0.6.0 to 10.0.7.255
network 10.0.6.0
broadcast 10.0.7.255
usable 10.0.6.1 to 10.0.7.254Things that catch people out
- 10.0.7.0/23 is not a network address, the block containing it is 10.0.6.0/23.
- After the merge, 10.0.6.255 is an ordinary usable host address, not a broadcast address. Firewall rules and monitoring that assumed .255 was broadcast will be wrong.
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 /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.
What a /27 subnet is: an eighth of a /24
A /27 uses the mask 255.255.255.224 and gives 30 usable addresses. What it is for and where it goes wrong.
What a /28 subnet is: sixteen addresses
A /28 uses the mask 255.255.255.240 and gives 14 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