How this instrument works
CIDR (Classless Inter-Domain Routing) notation writes an IP network as an address followed by a slash and a number, like 192.168.1.10/24 — that trailing number is the prefix length, the count of leading bits in a 32-bit IPv4 address that are fixed as the 'network' portion, with the remaining bits free to identify individual hosts within that network. A /24 fixes the first 24 bits (the first three octets), leaving 8 bits — 256 possible values — for host addresses within that block.
From the prefix length alone, you can derive everything else about the block: the subnet mask (a dotted-decimal pattern of 1-bits then 0-bits matching the prefix), the network address (the lowest address in the block, with all host bits zeroed), the broadcast address (the highest address, with all host bits set to one), and the number of usable host addresses in between. For most prefix lengths, the network and broadcast addresses themselves are reserved and not assignable to a device, so usable hosts equals total addresses minus 2.
Two prefix lengths get special handling under RFC 3021: a /31 (only 2 addresses total) has no reserved network or broadcast address at all — both addresses are usable, a deliberate exception designed for point-to-point links where reserving 2 of just 4 available addresses would be wasteful. A /32 is a single address with no room for a network at all, representing exactly one host.
- Enter the IPv4 address across the four octet fields (e.g. 192, 168, 1, 10 for 192.168.1.10).
- Enter the CIDR prefix length (0-32) — the number after the slash, like the 24 in /24.
- Read Subnet mask and Network address to see the block's lowest address and its dotted-decimal mask.
- Read Broadcast address, First usable host and Last usable host to see the block's full address range.
- Read Usable hosts and Total addresses to see how many devices the block can actually accommodate.
Worked example — 192.168.1.10/24, a typical home LAN
192.168.1.10 with a /24 prefix fixes the first 24 bits (192.168.1) as the network portion, leaving 8 bits (256 addresses) for hosts. The subnet mask is 255.255.255.0. The network address — the block's lowest address, with all host bits zeroed — is 192.168.1.0, and the broadcast address — the highest, with all host bits set — is 192.168.1.255.
That leaves 254 usable host addresses, from 192.168.1.1 (first usable host) through 192.168.1.254 (last usable host) — the network and broadcast addresses themselves, 192.168.1.0 and 192.168.1.255, are reserved and can't be assigned to a device. This is exactly the address range a typical home or small-office router hands out to connected devices.
Questions
What does the number after the slash in an IP address mean?
It's the CIDR prefix length — the count of leading bits in the 32-bit IPv4 address that are fixed as the network portion. A smaller number after the slash (like /8) fixes fewer bits and leaves more for hosts, meaning a much larger network; a larger number (like /30) fixes more bits and leaves very few for hosts, meaning a tiny network. /24 is the most common prefix for small office and home networks, giving 254 usable host addresses.
Why can't I use the network address or broadcast address for a device?
The network address (all host bits zero) identifies the subnet itself as a whole, and the broadcast address (all host bits one) is a special address that sends a packet to every device on that subnet simultaneously — neither represents a single, individually addressable device, so both are reserved by convention and can't be assigned to any one host. This is why usable hosts is normally 2 less than the block's total address count.
What's special about a /31 or /32 network?
A /32 has only one address total — it identifies a single specific host, with no room for a subnet at all. A /31 has exactly 2 addresses, and RFC 3021 makes both of them usable rather than reserving one as network and one as broadcast, since that reservation would waste half the tiny block — this exception exists specifically for point-to-point links (like a connection between two routers) where only 2 addresses are ever needed.
How do I know how many host addresses a given prefix length provides?
Total addresses in the block equal 2 raised to the power of (32 minus the prefix length) — a /24 gives 2^8 = 256 total addresses, a /16 gives 2^16 = 65,536, and a /30 gives 2^2 = 4. Usable hosts is normally that total minus 2 (for the reserved network and broadcast addresses), except for /31 (both addresses usable) and /32 (a single host, no subnet).
Is CIDR notation the same thing as a subnet mask?
They describe the exact same information in two different formats — CIDR notation (like /24) is a compact prefix-length number, while a subnet mask (like 255.255.255.0) writes the same network/host boundary out as a full dotted-decimal address with 1-bits for the network portion and 0-bits for the host portion. This instrument shows both side by side, since different tools and contexts prefer one format or the other.