Online nmap network scanner

The Nmap mini-bot discovers open ports in the host you specify and/or if it's alive!! This is a very handy tool in debugging network - services problems. This is a limited capability edition.

E.g 212.123.12.76 or yahoo.com

Nmap ("Network Mapper") is a free and open source (license) utility for network discovery and security auditing.
More info on Nmap.