What a /25 subnet is: half a /24
A /25 holds 128 addresses, 126 usable, and splits a /24 down the middle at .128.
At a glance
- Subnet mask
- 255.255.255.128
- Wildcard mask
- 0.0.0.127
- Total addresses
- 128
- Usable hosts
- 126
- Block size in the last octet
- 128
- Host bits
- 7 of 32
In detail
The mask is 255.255.255.128. There are two /25s in a /24: one from .0 to .127 and one from .128 to .255, each with its own network and broadcast address.
Splitting a /24 into two /25s is the usual way to separate two trust levels, for example servers and management interfaces, without asking for a new allocation.
Remember that splitting costs you two addresses. A /24 has 254 usable, but two /25s have 126 each, so 252 in total. Every further split costs two more per new subnet.
Examples
10.0.5.0/25 network 10.0.5.0, broadcast 10.0.5.127, usable .1 to .126
10.0.5.128/25 network 10.0.5.128, broadcast 10.0.5.255, usable .129 to .254Things that catch people out
- 10.0.5.128 is a valid, ordinary network address for the second half. It looks like a host address and is not one.
- 10.0.5.127 becomes a broadcast address after the split. Anything statically assigned to it stops working.
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 /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 /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 /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.
What a /29 subnet is: eight addresses
A /29 uses the mask 255.255.255.248 and gives 6 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