Getting Started
CIDR notation in plain language
CIDR (Classless Inter-Domain Routing) is the standard way to describe a
block of IP addresses. It looks like this: 192.168.0.0/24.
The part before the slash is the starting address. The number after the
slash tells you how many bits are locked as the "network" portion.
The remaining bits are yours to assign to devices. A /24
locks the first 24 of 32 bits, leaving 8 bits free — that gives
you 28 = 256 addresses. A /16 locks 16 bits and
leaves 16, giving you 65,536 addresses.
The key idea: prefix length controls block size
Every time you add one bit to the prefix, you cut the block in half.
A /24 has 256 addresses. A /25 has 128.
A /26 has 64. This halving is exactly what the
Split button does in slashwhat — it takes one
block and divides it into two equal halves by increasing the prefix
length by one.
Common prefix lengths
| Prefix | Addresses | Netmask | Common Use |
|---|---|---|---|
/8 | 16,777,216 | 255.0.0.0 | Large enterprise or ISP allocation |
/16 | 65,536 | 255.255.0.0 | Campus or regional network |
/20 | 4,096 | 255.255.240.0 | Cloud VPC default (AWS, GCP) |
/24 | 256 | 255.255.255.0 | Standard LAN subnet |
/28 | 16 | 255.255.255.240 | Small server segment |
/30 | 4 | 255.255.255.252 | Point-to-point link |
/32 | 1 | 255.255.255.255 | Single host route |
IPv6 works the same way
IPv6 addresses are 128 bits instead of 32, but the CIDR notation is
identical. 2001:db8::/48 means the first 48 bits are the
network, leaving 80 bits for hosts. slashwhat handles both IPv4 and
IPv6 — just type either format into the input.
Walk through your first split
This guide uses 192.168.0.0/24 as an example — a
standard 256-address subnet that most network engineers work with daily.
Step 1: Enter a CIDR block
Type 192.168.0.0/24 into the input field at the top of
the page and press Enter. The app creates a table with a single row
showing the full /24 block: 256 total addresses, 254 usable (the
network and broadcast addresses are reserved).
Step 2: Split
Click the green Split link on the right side of the row. The /24 divides into two /25 subnets:
192.168.0.0/25— 128 addresses (0 to 127)192.168.0.128/25— 128 addresses (128 to 255)
Each half is exactly half of the original. The math is always precise because splitting just adds one bit to the prefix length.
Step 3: Split again
Click Split on the first /25. It becomes two /26 subnets with 64 addresses each. You can keep splitting as deep as the address space allows — all the way down to /32 (a single host).
Step 4: Join
See the red Join bar on the left side of two sibling subnets? Click it to merge them back into their parent. Joining is the reverse of splitting — two /26 siblings merge back into the /25 they came from. You can only join subnets that share the same parent.
The tree structure
Behind the table, the app maintains a binary tree. Your original /24 is the root. Each split creates two children. Each join removes two children and restores the parent. This tree structure guarantees that every split is mathematically valid — no overlapping subnets, no gaps, no wasted addresses.
What you can do
Name your subnets
Click the Name field on any row and type a label — "Servers", "Guest Wi-Fi", "Management". Names appear in the table and are included when you save or export. Child subnets can inherit names from their parent for hierarchical labeling.
Multiple trees
Add more CIDR blocks by typing them into the input. Each becomes its
own section in the table. You can have 10.0.0.0/8 and
172.16.0.0/12 side by side, each with independent
split/join trees.
Save, Load, and Export
Save downloads your entire configuration as a JSON file. Load restores it later. Export produces a CSV file you can open in a spreadsheet. Everything is stored locally — nothing is sent to a server.
Undo and Redo
Made a mistake? Press Ctrl+Z (or Cmd+Z on Mac) to undo. Ctrl+Shift+Z to redo. The app keeps 8 levels of undo history in memory.
Color themes
Below the table you will find color controls. Choose from 17 themes (Pastel, Neon, Forest, Ocean, and more) and 7 coloring modes (by depth, by block size, cycle, alternating, and others). Switch between dark and light mode with the toggle in the header.
Column visibility
Hover over the column header row to reveal controls for hiding, showing, and reordering columns. Drag columns to rearrange them. Hidden columns can be restored from the "Hidden Columns" box that appears in the header area.
VLAN column
The VLAN column supports a macro language for automatic VLAN
assignment. Write expressions like 100 + $3 (where
$3 is the third octet) and the tool evaluates them
for each row. Presets are available for common VLAN schemes.
Tips and reference
Common subnet sizes for different use cases
| Use Case | Suggested Prefix | Addresses |
|---|---|---|
| Home or small office | /24 | 256 |
| Server VLAN (small) | /27 | 32 |
| Point-to-point link | /30 or /31 | 4 or 2 |
| Management network | /28 | 16 |
| Guest Wi-Fi | /23 | 512 |
| Cloud VPC | /16 to /20 | 4,096 to 65,536 |
| Data center supernet | /12 to /8 | 1M to 16M |
IPv6 support
slashwhat works with IPv6 CIDR blocks. Type an IPv6 prefix like
2001:db8::/48 and split just like you would with IPv4.
The table adapts to show full IPv6 addresses, prefix lengths, and
the much larger address counts.
Keyboard shortcuts
| Shortcut | Action |
|---|---|
Ctrl+Z / Cmd+Z | Undo |
Ctrl+Shift+Z / Cmd+Shift+Z | Redo |
Enter | Submit CIDR input |
Learn more
Read the About page for the full story of how slashwhat was built — the architecture, the AI coding process, and war stories from development.