IPv4 Subnetting Calculator
Enter one IPv4 address and CIDR prefix to see the enclosing block and its carefully qualified host or endpoint meaning.
- Subnet mask
- Block lower address
- Block upper address
- Total addresses
Host or endpoint interpretation
The IPv4/CIDR value you enter and its results stay in this browser. No lookup or upload is needed. After the page loads, a new CIDR can be calculated and exported offline.
Read the block before assigning addresses
Enter an address such as 192.168.1.130/26. The result identifies its containing network (192.168.1.128/26), mask (255.255.255.192), lower and upper block addresses, and total address count (64). In a conventional /26, the interior 192.168.1.129–192.168.1.190 has 62 theoretical host addresses. This is arithmetic, not a live claim that any address is available or safe to configure.
Why /31 is different
A /31 has just two addresses. RFC 3021 treats both as host endpoints when the /31 is used on a point-to-point link; a directed broadcast is not used on that link. This calculator therefore calls them lower and upper endpoints, rather than calling the upper address a broadcast. Other link types and local network policy may have different assignment rules.
Other boundary cases
A /32 is a one-address host route without a separate directed broadcast or conventional interior range. A /0 is the whole IPv4 default-route block, from 0.0.0.0 through 255.255.255.255; its 4,294,967,296 mathematical addresses are not one deployable subnet. RFC 4632 defines CIDR prefix notation and these block sizes. This tool does not test ownership, routing, IPv6, VLSM planning or provider-reserved addresses.
Frequently asked questions
What does an IPv4 /26 prefix mean?
A /26 keeps 26 network bits and leaves six address bits. That block has 64 total IPv4 addresses. For a conventional subnet, two boundary addresses are identified as network and directed broadcast, leaving 62 theoretical interior addresses; actual assignments depend on network policy.
Can both addresses in a /31 be used?
RFC 3021 interprets both /31 addresses as host endpoints when the /31 is used on a point-to-point link. In that case there is no directed broadcast to the link. This does not promise the same use on every other link type.
What is a /32 host route?
A /32 names one IPv4 address. It has one total address and no separate conventional host range or directed broadcast address. Its presence in a calculation does not say whether it is globally routable.
Does a /0 mean I can use 4,294,967,296 hosts?
No. A /0 spans the full IPv4 address space as a default-route block. Its 4,294,967,296 addresses are a mathematical block size, not one deployable subnet or assignable-host inventory.
Does this tool test a live network or allocate addresses?
No. It calculates unsigned IPv4/CIDR boundaries from the text you enter. It does not query routing tables, DNS, an address registry, a cloud provider or your network administrator, and it does not plan VLSM or IPv6 subnets.