DNS Lookup & Subnet Calculator

about this tool

Two tools, one page

A DNS lookup and a subnet calculator are the two things reached for when something on a network is not behaving. One needs a resolver; the other is arithmetic.

The lookup

The query is made by this site's own server, not by your browser. That matters: a page that queries DNS from the browser hands the resolver a request tied to your address. Here the resolver sees this server. The resolver used is Cloudflare's, which already serves this site, so no company is involved that was not already; the endpoint only accepts the ten record types the tool offers, so it is not a general proxy, and it neither logs nor stores anything.

Names are checked before anything is sent: labels of 1 to 63 characters, 253 in total, letters, digits and hyphens, and no hyphen at the start or end of a label, following RFC 1035 and RFC 1123. International names must be given in their punycode form, because that is what the DNS carries.

Answers are shown with their time to live in words, and the structured records are taken apart: an MX record's preference and mail server, an SOA's serial and timers with the responsible mailbox written as an email address, an SRV record's priority, weight, port and target, a CAA record's tag and value. Long TXT records, which arrive as several quoted strings, are joined back together. When there is nothing to show, the reason is given: a name that does not exist reads differently from a name with no record of that type, and the tool says which.

The answer comes through a resolver, so it is only as fresh as the record's time to live allows.

The calculator

Give it a network as 192.168.1.0/24, as a bare address, or with a dotted mask, and it works out the network address, the range, the netmask and its wildcard, the broadcast address and how many addresses there are — with the usable count worked out properly rather than always subtracting two. A /31 is a point-to-point link with two usable addresses and no broadcast (RFC 3021), a /32 is one host, and IPv6 subtracts nothing because it has no broadcast address. A mask with a hole in it, such as 255.0.255.0, is refused, since no prefix length can express it.

Everything is computed on the address as one big integer, so IPv6 works exactly as IPv4 does: a /48 correctly reports 1,208,925,819,614,629,174,706,176 addresses rather than overflowing. The splitter cuts a network into equal smaller ones, listing the first few and counting the rest.

Limits

The lookup asks a recursive resolver, not the authoritative name servers, so it cannot show what a zone says before caches expire, and there is no trace, no WHOIS and no reverse lookup by address — though the IP inspector builds the in-addr.arpa name a reverse lookup would use, which can be pasted in here as a PTR query. The calculator does not do variable-length subnetting across a list of required sizes; it splits a network evenly.

To see the address your own connection arrives from, use my IP & IP inspector — the other tool here answered by this site rather than your browser. The user-agent parser reads the other half of what a server learns about a visitor.

questions

Who sees my lookup?

This site’s own server makes the query, so the resolver sees this server rather than you. It asks Cloudflare’s resolver, which already serves this site, so no company is involved that was not already. Nothing about the query is logged or stored here.

Why does a record I just changed still show the old value?

Because a resolver holds an answer for as long as the record’s time to live says it may, and the answer here comes through one. Each record shows its remaining time to live. Only the authoritative name servers, which this tool does not query directly, always have the current value.

How many usable addresses are in a /31?

Two. Ordinarily an IPv4 subnet loses its first address to the network and its last to the broadcast, but RFC 3021 makes a /31 a point-to-point link with both addresses usable and no broadcast. A /32 is a single host. IPv6 has no broadcast address at all, so nothing is subtracted from any prefix.

Can I give it a mask instead of a prefix?

Yes: 192.168.1.0/255.255.255.0 is read the same as /24. A mask whose ones and zeros are mixed up, such as 255.0.255.0, is refused, because no prefix length can express it.

Why will it not look up an international domain?

The DNS carries names in punycode, the ASCII form, so a name like bücher.example has to be converted to xn--bcher-kva.example before it can be asked for. The tool says so rather than sending something the resolver cannot use.