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To calculate an IPv4 subnet, combine the address with its CIDR prefix: the prefix determines the mask and the block boundaries, which reveal the network address and—on an ordinary broadcast-capable subnet—the broadcast address and conventional host range. For IPv6, use the 128-bit prefix; IPv4 dotted masks and broadcast rules do not apply.
What a subnet calculator tells you
A subnet calculator takes an IP address and a prefix length such as /24, then identifies the block containing that address. For IPv4, useful results include the subnet mask, total addresses, network address, broadcast address, and conventional host range. Those results depend on the prefix, not on whether an address was once described as Class A, B, or C: modern CIDR uses explicit prefix lengths. RFC 4632 defines this classless approach, while RFC 1878 tabulates IPv4 masks and subnet ranges.
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How to calculate an IPv4 subnet
- Read the prefix. In
192.168.99.0/24,/24means the first 24 bits of the 32-bit IPv4 address are the network prefix. - Convert the prefix to a mask. Set that many leading bits to 1 and the rest to 0. A
/24mask is255.255.255.0; a/26mask is255.255.255.192. - Find the block size and boundary. A
/26leaves 6 bits for addresses, so each block contains 64 addresses. In the fourth octet, the block boundaries are 0, 64, 128, and 192. - Locate the input address within its block. For
192.168.1.70/26, 70 lies in the 64–127 block. - Read the network and range. The network is
192.168.1.64; the final address is192.168.1.127. On an ordinary broadcast-capable IPv4 subnet, those are the network and directed broadcast addresses, leaving192.168.1.65–192.168.1.126as the conventional host range.
A /24 contains 256 total addresses, while a /26 contains 64. The counts follow from the number of bits left after the prefix: 2^(32 − prefix length). The IETF’s RFC 4632 and RFC 1878 provide the corresponding IPv4 prefix and subnet tables.
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IPv4 host counts and special prefixes
For many conventional IPv4 subnets, subtracting the network and broadcast addresses from the total gives the conventional host count. That shortcut does not apply to every prefix.
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- Most ordinary broadcast-capable subnets: the first address identifies the network and the last is the directed broadcast address, so the addresses between them are conventionally available to hosts.
/31point-to-point link: both addresses in the two-address block can be used as host addresses. Do not subtract two and conclude that it has no usable hosts. This special use is specified by RFC 3021./32host route: the prefix identifies one address, not an ordinary subnet with a broadcast address and a range of host addresses. RFC 4632 describes the host-route use.
Advice to avoid all-zero and all-one subnets is outdated. RFC 1878 says, in the context of excluding those subnets, “This practice is obsolete!” Standards-based subnetting includes all definable subnets.
How IPv6 prefixes differ
IPv6 addresses are 128 bits long and use hexadecimal groups with slash-prefix notation. A prefix such as /64 identifies the leading 64 bits; IPv6 does not use an IPv4-style dotted-decimal netmask or broadcast address, and the ordinary IPv4 “subtract two” host-count rule should not be carried over. RFC 4291 defines IPv6 addressing.
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For globally unique and unique-local IPv6 subnets, RFC 5375 describes /64 as the subnet length and warns that other lengths can break features. IPv6 text can have multiple equivalent spellings; RFC 5952 recommends a consistent representation. A calculator that supports IPv6 should show the prefix and network clearly, without presenting IPv4 broadcast or host-range fields as if they applied.
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Choosing a prefix or checking a calculator
Choose a subnet size by matching the address family and prefix to the number of addresses needed, the network boundaries, and any special requirements. For IPv4 point-to-point links, account for /31 behavior; for globally unique and unique-local IPv6 subnets, account for the /64 convention. When checking a calculator, confirm that it validates the input and displays the network boundary and range correctly, handles IPv4 /31 and /32 appropriately, and supports IPv6 formatting if needed. A tool that explains its calculation makes it easier to catch an incorrect prefix or misunderstood result.
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- High Efficiency Visual Fault Locator: Easy identification of fiber breakpoints, poor connections, bending or cracking. Excellent for finding the right fiber to splice or quickly finding a break. Emmiting Energy: standard wavelenth: 650nm. Fast flashing, slow flashing, high precison.The built-in self-calibration ensures stable long-term performance, and Class IIIa laser (output<5mW) ensures safe daily operation.
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- PoE Testing and Cable Length Test: PoE testing can check cable mapping polarity and voltage of PoE network switches, withstand 60VDC. Automatically detects and switches between 10M/100M/1000M modes, Includes cable tracking, short circuit test, interruption of circuit test and etc The RJ45 cable tester can quickly measure the length of the cable with a range of 200m. Not only network cables, but also phone lines and BNC cables.
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