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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DNS changes not propagating: TTL, cache and delegation troubleshooting
Separate authoritative data, recursive caches, TTLs and delegation mistakes when a DNS change appears correct in one place but stale elsewhere.
01
Ask the authoritative server first
Before discussing propagation, query the authoritative nameserver directly. If it still returns the old record, the issue is publishing or zone configuration. If it returns the new value, the remaining difference is likely cache, delegation or a different authoritative server serving inconsistent data.
02
TTL controls cache lifetime, not global synchronization
Recursive resolvers may legally retain an answer until its TTL expires. Lowering the TTL after the old answer has already been cached does not shorten the lifetime of that cached copy. Plan TTL reductions before scheduled changes.
03
Check every authoritative nameserver
A zone with multiple authoritative servers can look intermittent when only some nodes loaded the new version. Compare SOA serials and the changed record against each nameserver rather than relying on a single public resolver.
04
Separate delegation problems from record problems
The parent zone may point to old nameservers, glue can be stale or DNSSEC delegation can disagree with the child zone. When the authoritative answer is correct but clients still reach the wrong infrastructure, verify the delegation chain.
05
Negative caching can keep “not found” answers alive
If a name was queried before it existed, resolvers may cache the negative result according to SOA and negative-caching rules. Creating the record immediately afterwards does not force every recursive resolver to forget the earlier NXDOMAIN.
06
CDNs and application caches can mimic DNS staleness
A browser, operating system, proxy or CDN may cache content independently of DNS. Compare the actual resolved address and authoritative answer before assuming every stale page is a DNS problem.
07
Record the old TTL before planned migrations
The safest change plan lowers TTL well before the cutover, waits for the old TTL horizon to pass, then changes the record. After the migration, restore a sensible TTL once rollback risk has passed.
EX
Worked example
- Query the authoritative nameserver and save the new answer plus TTL.
- Query every authoritative server and compare SOA serials.
- Query two unrelated recursive resolvers and compare remaining TTLs.
- If authoritative data is correct everywhere, inspect delegation, local OS cache and application/CDN cache.
✓
Operational checklist
- Authoritative answer is correct.
- All authoritative servers agree.
- SOA serial is consistent.
- Old TTL horizon is understood.
- Delegation and glue point to the intended servers.
Tools to apply it
Standards and references
🕐 History