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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Wildcard masks for OSPF and ACLs
Understand inverse masks, matching semantics and practical wildcard examples for routing and access-control work.
01
A wildcard is an inverse matching mask
Where a subnet mask uses ones for network bits, a classic wildcard uses zeros for bits that must match and ones for bits that may vary. For a /24 mask 255.255.255.0, the corresponding wildcard is 0.0.0.255.
02
OSPF and ACLs use the same bits for different jobs
The arithmetic is the same, but operational meaning depends on the command. In an ACL, the wildcard controls which source or destination addresses match a rule. In routing configuration it can define which interface addresses are selected by a statement.
03
Non-octet examples matter
Prefixes such as /27 or /21 are where manual mistakes become common. Convert the subnet mask first, invert every octet, then test the first and last matching addresses rather than trusting visual intuition.
04
Rule order still matters
A correct wildcard does not guarantee a correct policy. ACL processing order, implicit denies and platform-specific behavior must still be reviewed around the generated match.
05
Use binary when decimal stops being intuitive
For a /27, the final mask octet is 224 or 11100000 in binary; the wildcard is 31 or 00011111. Reading the bits makes it obvious which address bits must match and which may vary, and it scales better than memorizing decimal wildcard tables.
06
Know when non-contiguous wildcards are possible
Some ACL engines allow non-contiguous wildcard bits, creating patterns that are not equivalent to a normal CIDR prefix. That can be powerful but also hard to audit. Routing network statements and vendor syntax may impose different restrictions, so do not assume every wildcard accepted in one feature is valid in another.
07
Validate the match set, not just the arithmetic
Before deploying a rule, test a known matching address, the first address outside the intended range and any sensitive adjacent subnet. The operational question is whether the resulting set is correct, not simply whether 255 minus the mask produced the expected decimal.
EX
Worked example
- Take 192.168.10.64/27, whose subnet mask is 255.255.255.224.
- Invert each octet to obtain 0.0.0.31.
- The match covers 192.168.10.64 through 192.168.10.95 when the base address is aligned correctly.
- Test .63 and .96 as negative controls before using the expression in an ACL or routing statement.
✓
Operational checklist
- Convert the CIDR to a subnet mask first.
- Invert all four octets, not only the changing one.
- Check first, last and one out-of-range address.
- Review rule order and implicit policy separately from the wildcard.
Tools to apply it
Standards and references
🕐 History