user@netcalc ~/calc $
ipv4_calculator
Calculate network, broadcast, mask, hosts, class and binary for any IPv4 address
ip:
mask:
✓ stdout — ipv4_calculator
Enter runs the calculation · Shift+Enter inserts a new line
user@netcalc ~/calc $
ipv6_calculator
Calculate network info for IPv6 addresses in CIDR notation
ipv6:
pfx:
✓ stdout — ipv6_calculator
user@netcalc ~/calc $
vlsm_calculator
Variable Length Subnet Mask — assigns optimal blocks for each network segment
net:
pfx:
// Subnets required
name:
hosts:
name:
hosts:
name:
hosts:
✓ stdout — vlsm_calculator
| # | Name | Hosts Req | Hosts Avail | Prefix | Network | First Host | Last Host | Broadcast | Mask |
|---|
user@netcalc ~/calc $
usable_ip_range
Shows the exact usable host range for a given network
ip:
mask:
✓ stdout — usable_ip_range
user@netcalc ~/calc $
binary_ip_calc
Convert IP addresses to binary and vice versa, with per-octet visualization
dec:
bin:
✓ stdout — binary_ip_calc
user@netcalc ~/network $
cidr_converter
Convert between CIDR, decimal mask and wildcard. Includes full /1–/32 reference table.
in:
✓ stdout — cidr_converter
📋 CIDR Reference Table
| CIDR | Subnet Mask | Wildcard | Hosts | /24 subnets | Block |
|---|
user@netcalc ~/network $
wildcard_mask_calc
Calculate the wildcard (inverse) mask for Cisco ACLs, OSPF and firewall rules
mask:
✓ stdout — wildcard_mask_calc
user@netcalc ~/network $
supernet_calculator
Find the minimum supernet containing all specified networks (route summarization)
nets:
✓ stdout — supernet_calculator
user@netcalc ~/network $
subnet_splitter
Split a network into equal subnets by specifying a new prefix
net:
mask:
new:
✓ stdout — subnet_splitter
| # | Network | First Host | Last Host | Broadcast | Hosts | Mask |
|---|
user@netcalc ~/network $
range_to_cidr
Convert an IP range (start–end) to the minimum set of CIDR blocks
start:
end:
✓ stdout — range_to_cidr
| # | CIDR Block | Network | Broadcast | Hosts |
|---|
user@netcalc ~/network $
ip_to_integer
Convert IPv4/IPv6 addresses to integer and vice versa
ip:
int:
✓ stdout — ip_to_integer
user@netcalc ~/network $
ipv6_normalize
Expand and compress IPv6 addresses — normalize any valid format
ipv6:
✓ stdout — ipv6_normalize
user@netcalc ~/network $
hex_ip_converter
Convert between hexadecimal and IP address — useful for Windows route tables and Cisco logs
ip:
0x:
✓ stdout — hex_ip_converter
user@netcalc ~/network $
vlan_calculator
802.1Q VLAN info: tag encoding, PCP, DEI, VID, range and compatibility
vlan:
pcp:
dei:
✓ stdout — vlan_calculator
user@netcalc ~/dns $
reverse_dns_generator
Generate the PTR in-addr.arpa zone for an IPv4 network
net:
pfx:
dom:
✓ stdout — reverse_dns_generator
user@netcalc ~/dns $
ipv6_ptr_generator
Generate the PTR ip6.arpa record for an IPv6 address
ipv6:
host:
✓ stdout — ipv6_ptr_generator
user@netcalc ~/dns $
ip_geolocation
Locate a public IP or resolve a domain and geolocate its public address
Domains are resolved through DNS first; geolocation is based on the resulting public IP.
geo:
✓ stdout — ip_geolocation
user@netcalc ~/dns $
ip_address_planner
Interactive IP address space planner — click cells to edit, export to CSV
📋 IP Address Plan
| # | Name | Network | Mask/CIDR | Gateway | VLAN | Location | Notes |
|---|
user@netcalc ~/legal $
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DHCP pool exhaustion and address conflicts: diagnosis and prevention
Find lease exhaustion, stale reservations, rogue DHCP servers and overlapping static ranges before users lose connectivity.
01
Measure free leases before changing the subnet
A full scope can mean genuine growth, leases that are too long, abandoned reservations or clients repeatedly consuming addresses. Start by measuring active leases, free addresses and lease churn so the fix matches the cause.
02
Keep static infrastructure outside the dynamic pool
Printers, gateways, APs and manually configured hosts should not silently occupy addresses the DHCP server believes are available. Define an explicit static or reserved region and document exclusions.
03
Treat duplicate offers as a security and operations problem
A rogue or forgotten DHCP server can hand out a different gateway, DNS server or overlapping address range. Packet capture at the client side quickly reveals multiple DHCPOFFER sources and is more reliable than guessing from symptoms.
04
Use the right subnet size instead of extending leases forever
When legitimate client count approaches capacity, redesigning the prefix is safer than masking the problem with shorter leases. Make sure routing, VLAN, ACL and monitoring systems are updated together with the larger scope.
05
Watch lease churn and device identity behavior
Randomized client identifiers, imaging workflows or unstable devices can create more leases than physical users suggest. Look for repeated devices that appear under different identifiers before simply enlarging the network.
06
Match lease duration to client behavior
Guest Wi-Fi with high turnover may need a shorter lease than stable office desktops. Extremely short leases increase DHCP traffic and do not solve a fundamentally undersized subnet.
07
Monitor free capacity before users notice
Alerting on percentage of free leases and failed allocations turns an outage into a capacity-planning task. Combine that with documentation of static exclusions and reservations.
EX
Worked example
- A /24 provides 254 classic usable addresses before reservations.
- Reserve gateway and infrastructure outside the dynamic pool, then count the actual dynamic capacity.
- Compare active leases and free addresses during the busiest period.
- If demand consistently approaches the ceiling, plan a larger prefix or split clients across VLANs.
✓
Operational checklist
- Count active and free leases.
- Verify static exclusions and reservations.
- Capture DHCP offers to detect rogue servers.
- Review lease duration against client turnover.
- Alert before free capacity reaches zero.
Tools to apply it
Standards and references
🕐 History