Free network diagnostic tools

Check the state of any IP address or domain, instantly

WHOIS, DNS, IPv6, SSL and port checks — all free to use, no sign-up required.

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Tools

We will continue adding more tools related to networking, IP addresses, and domains.

Look up registrant information, expiry dates, and name servers for multiple domains at once.

Look up WHOIS information for a single domain.

Look up the allocated organization and network range of an IP address via WHOIS.

Look up DNS records from a domain to an IP address, or a hostname from an IP address.

Normalize IPv6 addresses, or convert IPv4 to IPv4-mapped IPv6 / 6to4.

Calculate the network address and number of usable hosts from CIDR notation (IPv4/IPv6 supported).

Check whether a specific port is open on a given host.

Ping a specified host and check its response time.

Trace the network route to a specified host.

Check the expiry date and issuer of an SSL/TLS certificate.

Check the HTTP response headers of a specified URL.

Check whether a domain supports IPv6 (has an AAAA record).

Estimate annual membership fees for APNIC, ARIN, RIPE NCC, LACNIC, and AFRINIC.

Monitor your URLs every 5 minutes and get notified by email when they go down (free, account required).

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Illustrated guides to how networks, IP addresses and domains work.

IPv6の省略表記ルールと、CIDR表記の読み方を図解。IPv4とIPv6でサブネットの考え方がどう違うのかを整理します。

WHOISはドメインやIPアドレスの登録情報を公開する仕組み。誰がどこから公開しているのか、何がわかって何がわからな…

DNS→Ping→Traceroute→ポートの順に確認すると原因の範囲を機械的に狭められます。各ツールの読み方と注意点を解説し…