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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When and why to use IPv4 /31 on point-to-point links
Learn why /31 is valid on point-to-point IPv4 links, what changes operationally and where it should not be used.
01
Why /31 works on point-to-point links
A traditional subnet reserves the all-zero and all-one host patterns for network and broadcast. On a true point-to-point link there are only two endpoints and no need for an IPv4 broadcast address, so RFC 3021 allows both addresses in a /31 to be used by the peers.
02
The operational benefit is address conservation
A /30 consumes four addresses to number two endpoints; a /31 consumes two. Across many routed links this halves the address space used for point-to-point infrastructure.
03
Where not to use it
Do not treat /31 as a universal replacement for small LANs. Ethernet segments with multiple hosts, legacy equipment or software that assumes classic broadcast semantics may require a conventional subnet.
04
Validate platform support
Modern routing platforms generally support /31, but the configuration should still be tested at both ends. Confirm ARP/neighbor behavior, routing adjacencies and monitoring before standardizing it.
05
Routing protocols behave normally when the link is truly point-to-point
OSPF, IS-IS over IP adjacencies, BGP sessions and static routes can operate across /31 addressing when the platform implements RFC 3021 correctly. The key requirement is that both ends understand the two addresses as host endpoints rather than network and broadcast addresses.
06
IPAM and monitoring must understand the prefix too
Older inventory systems may label one /31 address as network or broadcast even when routers use it correctly. Update validation, discovery and monitoring tooling so the address plan does not produce false alarms or refuse valid assignments.
07
Migrate deliberately from /30
When converting an existing /30, choose which two addresses will remain, update both ends during the same change window and verify routing adjacencies before reclaiming the unused addresses. The saving is valuable at scale, but a careless migration can create avoidable outages.
EX
Worked example
- A /30 such as 10.0.0.0/30 contains four addresses but normally uses only .1 and .2 as endpoints.
- With RFC 3021, 10.0.0.0/31 uses both .0 and .1 as endpoint addresses.
- Across 500 point-to-point links, /30 addressing consumes 2000 addresses while /31 consumes 1000.
- Verify the routers, IPAM and monitoring stack before standardizing the migration.
✓
Operational checklist
- Use /31 only on true two-endpoint links.
- Confirm support on both devices and management systems.
- Test routing adjacency and reachability in both directions.
- Document the convention so operations teams do not “fix” valid .0/.1 addresses.
Tools to apply it
Standards and references
🕐 History