When visiting a a web site produces a 404 or another message, check if it's that site that isn't working (down for everyone) or if it's your Internet service provider (just you).

RIPE's WHOIS service gives contact and registration information for IP addresses, autonomous system numbers (ASN), organizations or customers that are associated with these resources, and related Points of Contact (POC). It doesn't include information on domain names.

ARIN's WHOIS service gives contact and registration information for IP addresses, autonomous system numbers (ASN), organizations or customers that are associated with these resources, and related Points of Contact (POC). It doesn't include information on domain names or military networks.

Get detailed information about an Internet host. Enter an IP address (64.233.167.99), domain name (google.com), hostname (py-in-f99.google.com), or AS-number (AS15169) to get detailed dns information including reverse dns and forwards, whois information, who hosts the machine, and if it's listed in any RBLs (black lists).

Query DNS domain name servers for information about host addresses, mail exchanges, name servers, and related information

Check any Internet host's reachability and how much latency there is in communicating with it from 50 locations worldwide

Ping is commonly used to determine if a given host is 'alive' on the Internet. It will repeatedly send a datagram to a network host to elicit a response from it and show you if, and how long it took to respond.

Convert an IP address from 8/8/8/8-bit dotted-decimal format (dot-seperated, like 127.0.0.1) to 8/8/16-bit, 8/24-bit, and 32-bit notations in decimal, octal and hexadecimal.