Base-Network Summary
{{ subnetSummary.networkAddress }}/{{ cidr }}
Broadcast {{ subnetSummary.broadcastAddress }} {{ subnetSummary.totalNewNetworks }} Child Networks {{ subnetSummary.totalUsableAddresses.toLocaleString() }} Usable IPs
Network Usable Min Usable Max Broadcast
{{ s.networkAddress }} {{ s.usableMin }} {{ s.usableMax }} {{ s.broadcastAddress }} 

            

        


    



                

                

                    

                        

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IPv4 subnetting divides the 32-bit address space into logical network segments, letting administrators ration limited addresses, isolate traffic, and apply routing policies efficiently. Each subnet is identified by a network prefix expressed in Classless Inter-Domain Routing (CIDR) notation such as /24.

  
This calculator accepts an IP address, its current CIDR, and a target CIDR. A lightweight reactive engine derives the base network, then enumerates every equally sized child subnet, returning broadcast, usable-host, and summary counts instantly in table or JSON form.

  
Use it when restructuring office LANs, carving DMZs for IoT devices, or preparing audit evidence that each business unit owns a clearly scoped address block. *Check allocations against upstream policies before deploying in production.*






  
Technical Details:


  
  
Subnetting relies on a bitwise AND between an address and a subnet mask to reveal the network portion. Incrementing the prefix length shrinks host space geometrically, allowing hierarchical delegation. Key variables are the starting address A, current prefix p, and desired child prefix q where q > p.


  
  
    
    
  1. Convert A from dotted-decimal to a 32-bit integer.
    2. 
    
  2. Compute the base mask −1≪32−p.
    3. 
    
  3. Derive the network address N = A AND mask.
    4. 
    
  4. Set child block size S = 232 − q.
    5. 
    
  5. Iterate i = 0 … 2q−p − 1 generating Ni = N + S·i.
    6. 
  


  
  

    

      
        
LabelMeaning

      
      
        
networkAddressFirst address; identifies the subnet itself.

        
usableMinLowest assignable host address.

        
usableMaxHighest assignable host address.

        
broadcastAddressLast address; used for local broadcasts.

      
    

  

  
Values guide router configuration, DHCP scopes, and firewall rules. Hosts must stay within the usable range to avoid collisions.


  
  
    
    
  • IP address – dotted-decimal input, e.g. 192.168.0.1.
    • 
    
  • Current CIDR /p – prefix length 1–30.
    • 
    
  • Target CIDR /q – longer prefix 2–30 where q > p.
    • 
  


  
  

    
Split 192.168.0.1 /24 into /26:

    p=24,q=26
    S=232−26=64
    N₀=192.168.0.0,N₁=192.168.0.64 … N₃=192.168.0.192
  


  
  
    
    
  • Assumes classless routing; ignores legacy classful rules.
    • 
    
  • Counts exclude network and broadcast addresses per subnet.
    • 
    
  • No support for Variable-Length Subnet Masking in one pass.
    • 
    
  • Does not validate overlaps with external allocations.
    • 
  


  
  
    
    
  • Prefixes shorter than /1 or longer than /30 are rejected.
    • 
    
  • Address ending with .0 or .255 may still be valid inside narrow CIDRs.
    • 
    
  • Loopback 127.0.0.0/8 is syntactically valid but seldom subnetted.
    • 
    
  • Broadcast 255.255.255.255 is disallowed as input.
    • 
  


  
  
Concept validated by RFC 950 (Classless Inter-Domain Routing) and RFC 4632 (CIDR notation guidelines).


  
  
This calculation runs entirely in your browser; no address data leaves the device.






  
Step-by-Step Guide:

  
Follow these steps to derive child subnets and export them.

  
    
    
  1. Enter the IP address you want to subdivide.
    2. 
    
  2. Select the current CIDR that matches the existing block.
    3. 
    
  3. Choose the longer target CIDR for the desired subnet size.
    4. 
    
  4. Review the summary banner for base network, broadcast, and usable-host totals.
    5. 
    
  5. Switch between Table and JSON tabs to inspect details, then copy or download CSV/JSON as required.
    6. 
  






  
FAQ:


  

    Is my data stored?
    
No. All processing happens locally; nothing is transmitted or retained.

  


  

    Why exclude /31 and /32?
    
Those prefixes reserve zero or one host, making broadcast and usable-host calculations meaningless in typical subnetting scenarios.

  


  

    Can I subnet public IP ranges?
    
Yes, but ensure the resulting blocks are routed and registered with your provider to avoid reachability issues.

  


  

    Does it support IPv6?
    
No. IPv6 uses a different addressing model; use an IPv6-specific calculator.

  


  

    How many child networks are possible?
    
The tool returns 2q−p subnets, where q is the target prefix and p is the current prefix, up to 16,777,216 for /1 → /25.

  






  
Glossary:

  

    
CIDR
Prefix length notation indicating network bits.

    
Broadcast Address
Last address of a subnet used to reach all hosts.

    
Usable Host Range
Addresses assignable to devices between network and broadcast.

    
Subnet Mask
32-bit value separating network and host portions.

    
Loopback
Special 127.0.0.0/8 range directing traffic to the local host.

  







  
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