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Subnet Calculator
A free subnet calculator with three modes. IPv4 takes any address and a prefix from /0 to /32 and returns the network, broadcast, subnet mask, wildcard mask and usable host range. VLSM divides a network into right-sized subnets, largest first. IPv6 gives the network prefix, both address forms and how many /64s fit.
Network
192.168.1.0/24
Broadcast
192.168.1.255
First usable host
192.168.1.1
Last usable host
192.168.1.254
Subnet mask
255.255.255.0
Wildcard mask
0.0.0.255
Total addresses
256
Usable hosts
254
How to use this calculator
- Type an IPv4 address into the IP field. Any host inside a subnet works; the calculator finds the network.
- Type a CIDR prefix length (0 to 32) or pick a preset.
- Read the network address, broadcast, and usable host range. Verify your manual calculation matches.
- Use the VLSM tab to plan subnets of different sizes, or the IPv6 tab for IPv6 prefixes.
- For deeper coverage, read our subnetting guide for the manual fast method, or keep the subnet mask cheat sheet open for every prefix from /0 to /32.
Common subnet sizes
| CIDR | Subnet mask | Total hosts | Usable hosts | Typical use |
|---|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 | Home / small office LAN |
| /25 | 255.255.255.128 | 128 | 126 | Half a /24 |
| /26 | 255.255.255.192 | 64 | 62 | Department subnet |
| /27 | 255.255.255.224 | 32 | 30 | Small workgroup |
| /28 | 255.255.255.240 | 16 | 14 | Tiny segment |
| /29 | 255.255.255.248 | 8 | 6 | Server cluster |
| /30 | 255.255.255.252 | 4 | 2 | Point-to-point WAN link |
| /31 | 255.255.255.254 | 2 | 2 | P2P link (RFC 3021) |
| /32 | 255.255.255.255 | 1 | 1 | Single host (loopback) |
Subnet calculator FAQ
What is a subnet calculator?
A subnet calculator takes an IPv4 address and a CIDR prefix (like /24 or /26) and returns the network address, broadcast address, subnet mask, wildcard mask, usable host range, and total host count. It is the fastest way to verify subnetting math on the CCNA or CCNP exam, or in real network design work.
How do I calculate a subnet by hand?
Convert the CIDR to a host-bit count (32 minus the prefix length), then read the mask block size from the relevant octet. The network address is the IP with all host bits set to 0. The broadcast address has all host bits set to 1. Usable hosts are everything between, exclusive of network and broadcast. Our subnetting guide walks the full method.
What is the difference between a /24 and a /25?
A /24 has 256 total addresses and 254 usable hosts. A /25 splits that /24 in half: two /25 subnets, each with 128 addresses and 126 usable hosts. The first /25 starts at .0; the second /25 starts at .128. The block size in the fourth octet is 256 minus the mask, so a /25 has block size 128.
Does this subnet calculator support IPv6?
Yes. Switch to the IPv6 tab and enter an address and prefix length. It returns the network prefix, the compressed and expanded forms of the address, the last address in the prefix and how many /64 subnets fit. IPv6 has no broadcast address, and LAN subnets are almost always /64.
What is VLSM and how does the VLSM calculator work?
VLSM (variable length subnet masking) gives each subnet a mask sized to its own host count instead of one mask for every subnet. In the VLSM tab, enter the network to divide and the hosts each subnet needs. The calculator sorts them largest first and assigns each the smallest block that fits, which is how the CCNA expects you to do it by hand.
Are .0 and .255 always reserved?
Only inside a /24 or larger. The lowest address in any subnet is the network address; the highest is the broadcast. In a /26 from 192.168.1.0 to 192.168.1.63, the reserved addresses are .0 and .63, not .0 and .255. The /31 prefix is a special case used for point-to-point links where both addresses are usable (RFC 3021).
What is CIDR notation?
CIDR (Classless Inter-Domain Routing) notation writes a subnet as an IP address followed by a slash and a prefix length, like 192.168.1.0/24. The prefix length tells you how many bits identify the network; the remaining bits identify hosts. CIDR replaced the older Class A/B/C system in 1993.
Want to master subnetting end-to-end?
Subnetting is one topic on the CCNA exam, and the topic that trips most students. CISNET's live online CCNA bootcamp covers it in week one with hands-on labs and the in-your-head fast method.
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