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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GeoIP
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PMTUD and TCP MSS clamping explained
Understand why some sites fail across tunnels, how Path MTU Discovery works and when MSS clamping is appropriate.
01
PMTUD tries to discover the smallest hop MTU
Path MTU Discovery depends on the network returning information when a packet cannot be forwarded at its current size. When that feedback is filtered or broken, small packets work while larger application flows can stall.
02
The symptom often looks application-specific
DNS, ping and short HTTP responses may succeed while TLS handshakes, file uploads or some websites hang. This makes a path-MTU problem look like a firewall, browser or server issue.
03
MSS clamping can protect TCP sessions
Adjusting the advertised TCP MSS at a tunnel boundary can prevent peers from sending segments that would create oversized packets. It helps TCP, but it does not fix non-TCP traffic or the underlying PMTUD failure.
04
Test with deliberate packet sizes
Use controlled probes and actual application traffic. Validate both directions because asymmetric tunnels and policies can create different effective MTUs on the forward and return paths.
05
IPv4 and IPv6 fail differently
IPv4 can fragment in some circumstances, while IPv6 relies on endpoints to adapt and routers do not fragment transit packets. Both therefore depend on receiving the right ICMP feedback for robust PMTUD, but the operational symptoms and packet captures are not identical.
06
Remember UDP and other non-TCP traffic
MSS clamping only influences TCP. Large UDP datagrams, QUIC, IPsec payloads and other protocols can still hit the same path MTU ceiling. If clamping “fixes the web” but other traffic remains broken, the underlying PMTUD problem is still present.
07
Instrument the tunnel instead of guessing
Useful telemetry includes interface MTU, ICMP PTB counters, retransmissions, TCP MSS observed in SYN packets and packet-size probes from both sides. This turns an intermittent application complaint into a measurable path property.
EX
Worked example
- A client sends a 1500-byte packet into a path whose tunnel hop only carries 1420.
- The router must return the appropriate ICMP feedback; if that message is blocked, the sender keeps retrying packets that never fit.
- TCP MSS clamping can advertise a smaller segment size at the boundary so TCP avoids generating oversized packets.
- Still repair ICMP handling and test QUIC/UDP, because MSS clamping does not solve those flows.
✓
Operational checklist
- Verify ICMP PTB / Fragmentation Needed can return to the sender.
- Test both directions with controlled packet sizes.
- Use MSS clamping as a TCP mitigation, not a universal MTU fix.
- Include UDP/QUIC in validation when applications use them.
Tools to apply it
Standards and references
🕐 History