Subnet Ip Range Calculator
Knowing a subnet’s prefix is only half the picture. What you usually need for real work — DHCP scopes, firewall rules, documentation — is the actual range of addresses you can assign: the first usable host through the last usable host. Working that out by hand on a /25 or /27 is tedious and error-prone. The subnet IP range calculator above takes any IPv4 address and CIDR prefix and instantly returns the subnet ID, the full usable range, the broadcast address, and the host count.
How to Use the Subnet IP Range Calculator
- Enter the IP address. Type any address in the subnet, such as 172.16.8.130. It does not have to be the first address — any host in the subnet works.
- Enter the subnet prefix. Type the CIDR number, for example 25.
- Press Calculate. The tool derives the subnet boundaries and the assignable range.
- Read the usable IP range. This “first – last” pair is the pool you can assign to devices and DHCP scopes.
- Check the subnet ID and broadcast address. These bound the range and are reserved — never assign them to hosts.
- Press Reset to analyze another subnet.
Worked Example
Enter 172.16.8.130 with a /25 prefix:
- Subnet ID: 172.16.8.128 (the /25 splits the last octet into 0-127 and 128-255)
- First usable host: 172.16.8.129
- Last usable host: 172.16.8.254
- Broadcast address: 172.16.8.255
- Usable IP range: 172.16.8.129 – 172.16.8.254
- Usable host count: 126
Try 10.0.5.200 with a /28:
- Subnet ID: 10.0.5.192
- First usable host: 10.0.5.193
- Last usable host: 10.0.5.206
- Broadcast address: 10.0.5.207
- Usable IP range: 10.0.5.193 – 10.0.5.206
- Usable host count: 14
Enter both into the calculator to confirm. Notice that 10.0.5.200 falls in the 192-207 block, not the 208-223 block — the kind of boundary trap this tool eliminates.
More Helpful Information
How the range is found. The calculator locates the subnet ID by rounding the address down to the nearest multiple of the block size, then the broadcast is subnet ID + block size − 1. First usable is one above the subnet ID; last usable is one below the broadcast. On /31 links both addresses are usable, and a /32 has exactly one.
Block sizes to remember. Each prefix step halves the block: /24 = 256 addresses, /25 = 128, /26 = 64, /27 = 32, /28 = 16, /29 = 8, /30 = 4. In the last octet, a /28 starts new subnets every 16 addresses (0, 16, 32, … 240).
Practical uses. DHCP servers take the usable range as the scope. Firewall rules often whitelist a range like 10.0.5.193-10.0.5.206. Network documentation should always list the full range so the next engineer does not have to recompute it.
Common mistakes. Assigning the subnet ID or broadcast address to a host, assuming the range starts at .1 (it starts at the first host after the subnet ID, which may be .129 or .193), and forgetting that the gateway router usually takes one address out of the pool — typically the first or last usable one.
Growth planning. If your usable range is nearly full, do not squeeze — move to a shorter prefix. A /24 that is 90% full should become a /23, doubling the range cleanly.
Frequently Asked Questions
1. What is the usable IP range of 192.168.1.0/24?
192.168.1.1 through 192.168.1.254 — 254 usable hosts.
2. How do you calculate the first and last usable IP?
First usable = subnet ID + 1; last usable = broadcast address − 1.
3. What is the IP range of a /28?
16 total addresses, 14 usable — for example 10.0.5.193 to 10.0.5.206 within the 10.0.5.192/28 subnet.
4. Can I assign the broadcast address to a device?
No. The broadcast address is reserved for traffic to all hosts on the subnet.
5. What is the range of 172.16.8.130/25?
172.16.8.129 to 172.16.8.254, with subnet ID 172.16.8.128 and broadcast 172.16.8.255.
6. Why is the subnet ID not usable?
It identifies the network itself in routing tables; assigning it to a host creates conflicts.
7. How many usable IPs are in a /30?
2 usable hosts — typically used for point-to-point links between routers.
8. Does any address in the subnet give the same range?
Yes. Any host address with the correct prefix produces the same subnet ID, range, and broadcast.
9. What is the difference between this and a subnet ID calculator?
A subnet ID calculator finds the network address; this tool goes further and lists every assignable address between the boundaries.
10. What range does a /31 give?
Both addresses are usable (per RFC 3021), so a /31 range is simply the two addresses, e.g. 10.1.1.0 – 10.1.1.1.
11. How do I size a DHCP scope from this?
Use the usable range, then exclude any statically assigned addresses such as the gateway, printers, and servers.
12. Why do ranges sometimes start at odd numbers like .193?
Because smaller prefixes split octets at non-.0 boundaries — a /28 divides the last octet into blocks of 16.
13. Can ranges overlap between VLANs?
They must not. Overlapping ranges on different VLANs cause duplicate-IP conflicts when traffic crosses.
14. Is the gateway inside the usable range?
Yes. The gateway is normally the first or last usable host address in the subnet.
15. How do I document a subnet properly?
Record the subnet ID with prefix (e.g. 10.0.5.192/28), the usable range, the gateway, and the broadcast address.
CONCLUSION
The usable IP range is the working heart of any subnet, and this calculator hands it to you — first host, last host, broadcast, and count — from a single address and prefix. Use it to build DHCP scopes, write firewall rules, and document networks without second-guessing boundary math. Keep the block-size table handy, never assign the boundary addresses to hosts, and your subnets will be clean, conflict-free, and ready to grow.