Net Tools

Twenty-four networking tools, and not one of them asks anybody anything

The whole group was chosen for what it leaves out. Every tool here answers a question that can be answered from the text in front of it — there is no lookup, no resolution, no probe and no port scan, because a page that did any of those would be sending your internal addressing to somebody's server and would stop working the moment you were offline. What is left is the arithmetic, and the arithmetic is the part that is easy to get wrong by hand.

Blocks first: a subnet calculator that shows the bits, an IPv6 one that counts the whole address space exactly rather than one short of it, a range turned into the smallest set of blocks that covers it, an address plan cut largest first so it does not fragment, a bulk check of which of your blocks each address falls in, and the four ways a v4 address is written inside a v6 one. Then one address at a time — every notation including the three that get past a filter, masks and wildcards with the mask that is *not* a prefix refused, and hardware addresses in all four styles.

Then names: zone-file lines with the mistakes named rather than left to a resolver, an SPF record with its ten DNS lookups counted before it stops working, DMARC with the advice everybody skips, a DKIM key split the way a TXT record has to be, and a domain in both its alphabets. Then what a link carries — bandwidth from your own table of rates, a latency budget with the line the speed of light takes, protocol overhead counted from the wire rather than the frame, and the TCP window a long link needs. Last, three things a protocol writes down, and a Wi-Fi code whose password never leaves the page.

Blocks of addresses

Subnet calculator
Network, mask, broadcast, host range and the bits behind them.

IPv6 subnet calculator
The range, both notations and how many networks fit inside.

CIDR range calculator
A range as the fewest blocks, blocks joined, and the overlap of two sets.

VLSM planner
One network cut into the subnets you asked for, largest first.

Address in subnet check
Which of your blocks each address falls in, and which fall in none.

IPv4 in IPv6
The four ways a v4 address is written inside a v6 one, both directions.

One address

IP address notations
Dotted, decimal, hexadecimal and binary — including the forms that get past filters.

Wildcard mask converter
Prefix, mask and wildcard, with the mask that is not a prefix refused.

MAC address tools
One notation out of four, what the two flag bits say, and a generated address.

Offline address lookup
Longest-prefix match in a table you paste. No table is shipped.

Names and their records

DNS record builder
Zone-file lines, with the mistakes named rather than left to a resolver.

SPF record builder
The record, with its ten DNS lookups counted before it stops working.

DMARC record builder
The policy, what each tag does, and why to watch before rejecting.

DKIM record formatter
A public key split into the strings a TXT record is made of.

Punycode converter
A domain in both alphabets, and a warning when one label mixes two.

What a link carries

Bandwidth calculator
What a link has to carry, from your own table of rates.

Latency budget
Where a response time goes, including the part the speed of light takes.

Packet overhead
What the headers cost and what is left for the payload.

TCP window calculator
The window a link needs to be kept full, and when scaling is required.

Port number reference
A port by number or by name, with its range and what is worth knowing.

What a protocol writes

HTTP header explainer
What each header does, which are missing, and which give your software away.

Request header builder
A set of headers as a command, a block or code, with the secrets hidden.

WebSocket frame decoder
A hexadecimal frame into its fields, and a message back into a frame.

Wi-Fi QR code
A code that joins a network. The password never leaves this page.

Questions

Which tools from the group are missing?: The ones that are a *request* rather than a calculation: a ping, a traceroute, a DNS lookup, a port scan, a certificate check. Every one of those needs a server to run it, which would mean sending your addresses to somebody — and would stop the page working offline. This is where they are named rather than left for you to look for.
Why does an address calculator care about hexadecimal?: Because `0x7f.1`, `017700000001` and `2130706433` are all 127.0.0.1 to most of the C library, and that is exactly why they turn up in a URL meant to get past a filter that was looking for `127.`. Reading them is right; reading them silently is not, so the tool says which notation it just accepted.
Are the port and vendor lists complete?: No, and both say so. The IANA port register is fourteen thousand rows and the vendor prefixes are four megabytes; what is here is the hundred-odd ports a person actually looks up, and no vendor table at all. A port that is not listed is not an unassigned port — and the *range* it falls in, well known or registered or ephemeral, is a fact about the number itself, so that answer is right either way.
Do the mail record builders check what is already published?: They cannot, because that would be a lookup. What they do is build the record and count what it will cost: an SPF record is allowed ten DNS lookups and fails *altogether* past them, so counting is the whole job. The rest is the advice the records need and rarely get — one SPF record a domain, and weeks of watching before a DMARC policy rejects anything.

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