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Enter CIDR notation such as 2001:db8:1200::/48 or fd12:3456:789a::/48.
Choose the per-site boundary; /56 gives 256 site prefixes from a /48 parent.
/
Use /64 for normal access networks; longer prefixes should be limited to special cases.
/
Enter the current or planned number of sites, branches, regions, or tenants.
sites
Count VLANs, routed segments, VRFs, service zones, and growth slots that need distinct LAN prefixes.
LAN prefixes
Use 0 for the first site prefix; large plans can preview a later allocation block.
Choose how many site prefixes to render in the visible ledger.
Use a short label such as Site, Branch, Region, Tenant, or POP.
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                            Tags: IP Addressing, Networking
                        

                        

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IPv6 prefix planning turns a delegated block into a repeatable address plan for sites, LANs, service zones, and growth. A parent such as /48 or /56 is usually too broad to describe one network segment. It becomes smaller child prefixes so routing, reverse DNS, firewall policy, and IP address management records all use the same boundaries.

    
The planning question is mostly about how many bits are reserved for each purpose. Site bits create separate site, branch, region, tenant, or point-of-presence allocations. LAN bits create routed segments inside each site. The remaining bits stay inside each LAN prefix for interface identifiers and address assignment.

    
A normal access LAN is still anchored around /64 because many IPv6 mechanisms expect a 64-bit interface identifier. Prefixes longer than /64 can be useful for special infrastructure cases, but they should not be treated as ordinary client LANs without checking the addressing method. Prefixes shorter than /64 usually describe aggregates that still need to be split before they become user or device segments.

    

      IPv6 address bits split into parent prefix, site identifiers, LAN identifiers, and interface identifiers
    

    
Good prefix plans keep capacity math separate from operational suitability. A plan can have enough child prefixes while still using a documentation block, a unique-local block, a link-local prefix, a non-nibble boundary, or a longer-than-/64 LAN size that needs extra review. Those details matter before a route, VLAN, DNS zone, or firewall object is copied into production notes.

    
The safest review order is parent block, site count, LAN count, /64 suitability, and clean hexadecimal boundaries. That order catches the common mistakes: counting addresses instead of prefixes, planning too many LANs inside each site, or accepting a mathematically valid prefix that is awkward for DNS and operations.

  






  

    
Technical Details:

    
IPv6 CIDR combines a 128-bit address with a prefix length. The prefix selects the fixed leftmost bits. Child planning then divides the remaining bits into site identifiers, LAN identifiers, and interface identifiers. The numeric count at each step is a power of two because every extra planning bit doubles the number of child prefixes available at that boundary.

    
Three prefix lengths define this plan. The parent prefix is the delegated or routed block. The site prefix is the per-site allocation size carved from that parent. The LAN prefix is the routed segment size inside each site. For the hierarchy to make sense, the site prefix must be equal to or longer than the parent prefix, and the LAN prefix must be equal to or longer than the site prefix.

    
The core arithmetic is compact. If a /48 parent is split into /56 sites, there are eight site bits, so the parent contains 2^8, or 256, site prefixes. If each /56 site is split into /64 LANs, another eight bits produce 256 LAN prefixes per site.

    

      
        
          
            siteBits
            =
            sitePrefix-parentPrefix
          
          
            sitePrefixes
            =
            2siteBits
          
          
            lanBitsPerSite
            =
            lanPrefix-sitePrefix
          
          
            lansPerSite
            =
            2lanBitsPerSite
          
          
            interfaceIdBits
            =
            128-lanPrefix
          
        
      
    

    

      

        
        
          

            Quantity
            How it is derived
            Planning meaning
          

        
        
          

            Site prefix capacity
            2^(site prefix - parent prefix)
            How many equal site, branch, region, tenant, or POP blocks fit in the parent.
          

          

            LAN capacity per site
            2^(LAN prefix - site prefix)
            How many equal LAN or routed segment prefixes fit inside each site block.
          

          

            Total LAN prefixes in parent
            2^(LAN prefix - parent prefix)
            The full count of child LAN prefixes across the parent when every site uses the same LAN size.
          

          

            Interface ID bits per LAN
            128 - LAN prefix
            The remaining address bits inside each LAN prefix after routing and subnet identifiers are fixed.
          

        
      

    

    
The canonical parent CIDR is calculated by clearing host bits below the parent prefix. That means an entered value such as 2001:db8:1200:1234::/48 is normalized to the actual parent boundary before capacity and ledger rows are built. Text form is then compressed in lowercase IPv6 notation so repeated entries are easier to compare.

    
Boundary choices also affect operations outside the arithmetic. A /64 LAN keeps the 64-bit interface identifier expected by common stateless address autoconfiguration designs. A prefix longer than /64 creates more routed prefixes but leaves fewer than 64 interface bits. A prefix shorter than /64 describes a larger aggregate that can contain multiple normal LANs.

    

      

        
        
          

            Check
            Trigger
            What to do with it
          

        
        
          

            Prefix order
            Site prefix shorter than parent, or LAN prefix shorter than site.
            Fix the hierarchy before trusting any capacity count.
          

          

            /64 LAN boundary
            LAN prefix is shorter or longer than /64.
            Use /64 for ordinary access LANs; reserve non-/64 sizes for deliberate cases.
          

          

            Hex nibble alignment
            Parent, site, or LAN prefix is not divisible by four.
            Review reverse DNS and human-readable allocation maps because hexadecimal boundaries no longer line up cleanly.
          

          

            Parent prefix scope
            Parent is documentation, ULA, link-local, global unicast, or special/reserved space.
            Confirm the address family matches the environment before treating the plan as deployable.
          

        
      

    

    
The site-prefix ledger is deterministic. Each visible site row advances by the site prefix size, then lists the first and last LAN prefix inside that site. The preview row limit changes how many ledger rows are shown, not the capacity math in the summary, metrics, guidance, chart, or JSON data.

  






  

    
Everyday Use & Decision Guide:

    
Start with the delegated parent exactly as it appears in the ISP handoff, IPAM record, lab plan, or ULA design note. If host bits are present, the plan will normalize the value back to the parent network and warn you, which is useful when a copied address came from a router interface instead of the allocation record.

    
For a common enterprise pass, use a parent such as /48, choose a site prefix such as /56, and keep LANs at /64. Then enter realistic demand in Sites needed and LANs per site needed. The result badges tell you whether the site count and LAN count fit before you spend time reading every ledger row.

    
    
      
  • Use Prefix Fit Metrics to check capacity, utilization, and interface ID bits.
    • 
      
  • Use Allocation Guidance when a fit badge reports a shortfall or a warning appears.
    • 
      
  • Use Site Prefix Ledger for exact per-site prefixes and first-to-last LAN boundaries.
    • 
      
  • Use Prefix Bit Budget when you need a quick visual check of parent, site, LAN, and interface bits.
    • 
      
  • Use Plan Inputs and JSON when the plan needs to be reviewed or stored with the same settings.
    • 
    

    
A fit result does not prove that the prefix is routed, assigned, or ready for production. The parent scope badge matters. A 2001:db8::/32 result is documentation space, a ULA result is private/local design space, and a link-local result cannot be treated like a routed site allocation.

    
Use the advanced ledger controls when the plan is large. Ledger start site index lets you preview a later allocation slot without changing total capacity. Ledger preview rows keeps the page readable. Site label prefix changes human labels such as Site, Branch, Region, Tenant, or POP while leaving the prefix math unchanged.

    
Before copying results into a change request, clear any red errors and read every yellow warning. The most important warnings are non-/64 LANs, non-nibble prefix boundaries, a normalized parent CIDR, and a ledger start index that was invalid or beyond the available site range.

  






  

    
Step-by-Step Guide:

    
Use the main fields for the allocation model, then open Advanced only when the visible ledger needs a different starting point or label.

    
    
      
  1. Enter Assigned parent prefix in IPv6 CIDR form, such as 2001:db8:1200::/48 or fd12:3456:789a::/48. If the prefix is invalid, the summary changes to Check prefixes and the error list names the format problem.
    2. 
      
  2. Set Site prefix length. The value must be equal to or longer than the parent prefix; otherwise the result reports that the site prefix cannot be carved from the parent.
    3. 
      
  3. Set LAN prefix length. Keep /64 for ordinary access networks, and check the warning area if the value is shorter or longer.
    4. 
      
  4. Enter Sites needed and LANs per site needed. The summary badges should report site fit and LAN fit before the plan is used as a handoff record.
    5. 
      
  5. Open Advanced when a large plan needs a later Ledger start site index, fewer Ledger preview rows, or a different Site label prefix.
    6. 
      
  6. Review Prefix Fit Metrics for capacity and utilization, then read Allocation Guidance for shortfalls, /64 suitability, nibble alignment, and parent scope.
    7. 
      
  7. Use Site Prefix Ledger to copy exact site and LAN boundaries, and use Prefix Bit Budget or JSON when another reviewer needs the bit split or machine-readable state.
    8. 
    

    
If a warning says the parent was normalized, compare the normalized parent CIDR against the source allocation before using the ledger rows.

  






  

    
Interpreting Results:

    
The headline count is the number of site prefixes available from the parent at the selected site prefix length. It is not the number of LANs, host addresses, or usable production sites. Read it together with Site utilization, LAN utilization per site, and Parent prefix scope.

    
Allocation Guidance is the main stop-and-fix section. If Site fit or LANs per site fit reports a capacity shortfall, the current prefix sizes do not meet the demand values you entered. A longer LAN prefix can create more routed prefixes inside a site, but anything longer than /64 should be reviewed before it is used for ordinary device networks.

    

      

        
        
          

            Output
            Read it as
            Double-check when
          

        
        
          

            Site prefix capacity
            Total equal site prefixes available from the normalized parent.
            The planned site count is close to the capacity or already short.
          

          

            LAN capacity per site
            Equal LAN prefixes available inside each site prefix.
            The value is below LANs per site needed or requires a non-/64 LAN size.
          

          

            Interface ID bits per LAN
            Remaining address bits after parent, site, and LAN bits are fixed.
            The value is not 64 for an access network.
          

          

            Hex nibble alignment
            Whether parent, site, and LAN boundaries align with hexadecimal digits.
            The status says Non-nibble boundary and reverse DNS delegation needs a clean zone cut.
          

          

            Parent prefix scope
            Whether the parent looks like documentation, ULA, link-local, global unicast, or special/reserved space.
            The plan will be used outside a lab, private ULA design, or example document.
          

        
      

    

    
The ledger rows are exact for the current settings, but they are still planning output. Confirm routing ownership, allocation policy, firewall conventions, DNS delegation, and naming standards before treating a row as an approved production prefix.

  






  

    
Worked Examples:

    
A documentation parent split for branch planning

    
With Assigned parent prefix set to 2001:db8:1200::/48, Site prefix length set to /56, LAN prefix length set to /64, Sites needed set to 120, and LANs per site needed set to 80, Prefix Fit Metrics reports 256 site prefixes and 256 LAN prefixes per site. The site plan has 136 spare site slots, and each site has 176 spare LAN slots.

    
The first Site Prefix Ledger row is Site 1 with site prefix 2001:db8:1200::/56, first LAN prefix 2001:db8:1200::/64, and last LAN prefix 2001:db8:1200:ff::/64. Parent prefix scope reports documentation prefix, so the arithmetic is useful for examples but the block must be replaced before production use.

    
A ULA plan with a non-nibble site boundary

    
Using fd12:3456:789a::/48 as the parent, a /57 site prefix, and a /64 LAN prefix creates 512 site prefixes and 128 LAN prefixes per site. If demand is 100 sites and 40 LANs per site, both fit, and Parent prefix scope reports ULA scope.

    
Allocation Guidance also reports Non-nibble boundary because /57 does not land on a 4-bit hexadecimal boundary. That does not invalidate the route plan, but it is a warning to review reverse DNS and any allocation map that depends on clean hex grouping.

    
A LAN shortfall inside each site

    
A 2001:db8:1200::/48 parent with /56 sites and /64 LANs can hold 256 LAN prefixes in each site. If LANs per site needed is 300, LANs per site fit reports a capacity shortfall and the metric row says the plan is short by 44 LAN prefixes per site.

    
For ordinary access LANs, the better correction is usually to give each site a broader site prefix, reduce the per-site LAN demand, or split the organizational model differently. Changing the LAN prefix to /65 creates more routed prefixes, but Allocation Guidance will warn that longer-than-/64 LANs are not suitable for ordinary SLAAC access networks.

  






  

    
FAQ:

    

      Why does the parent prefix change after I enter it?
      
The calculator clears address bits below the parent prefix and reports the network boundary. For example, a host-like value inside 2001:db8:1200::/48 is normalized back to the /48 parent before site and LAN capacity are counted.

    

    

      Should every LAN prefix be /64?
      
Use /64 for ordinary access networks unless you have a deliberate reason not to. The page warns when the LAN prefix is shorter or longer because non-/64 plans can change SLAAC behavior, documentation expectations, and operational review.

    

    

      What does a non-nibble warning mean?
      
At least one parent, site, or LAN prefix length is not divisible by four. The prefix can still be valid, but hexadecimal boundaries are harder to read and reverse DNS delegation may need extra care.

    

    

      Why does 2001:db8 show as documentation space?
      
2001:db8::/32 is reserved for examples and documentation. It is safe for training and planning examples, but it should be replaced with a real routed or delegated prefix before production deployment.

    

    

      Does the calculation send my prefix to a server?
      
The prefix math runs in the browser from the values you enter. The page may load supporting assets for the interface, and shared URLs can carry entered values in the address bar, so avoid sharing sensitive internal plans in copied URLs.

    

  






  

    
Glossary:

    

      
Parent prefix

      
The delegated or routed IPv6 CIDR block being divided into smaller allocations.

      
Site prefix

      
The child prefix size assigned to each site, branch, region, tenant, or similar allocation unit.

      
LAN prefix

      
The routed segment size inside each site prefix, usually /64 for ordinary access networks.

      
CIDR

      
Classless Inter-Domain Routing notation, written as an address plus a slash prefix length.

      
SLAAC

      
Stateless Address Autoconfiguration, a common IPv6 method that relies on the normal 64-bit interface identifier model.

      
Nibble alignment

      
A prefix boundary divisible by four, matching one complete hexadecimal digit at a time.

      
ULA

      
Unique Local Address space, usually used for private IPv6 communication inside a site or limited set of sites.

    

  






  

    
References: