The process of testing a remote server's open or listening ports is called "Scanning".
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The Internet is full with various scanner and network suit programs, and many security related websites offer a lot of information about these programs, therefore I've decided to only write about a small number of my favorite programs, and only if they're freeware.
(currently v3.0) My personal favorite! A powerful connect-based TCP port scanner, pinger and hostname resolver. Multithreaded and asynchronous techniques make this program extremely fast and versatile. Perform ping scans and port scans using any IP range or specify a text file to extract addresses from. Scan any port range from a built in list or specified range. Resolve and reverse-lookup any IP address or range. Modify the port list and port descriptions using the built in editor. Connect to any discovered open port using user-specified "helper" applications (e.g. Telnet, Web browser, FTP) and assign a custom helper application to any port. Save the scan list to a text file. User friendly interface.
Download it from HERE (197kb)
Angry IP scanner
(currently v2.19) - is a very fast and small IP scanner. It pings each IP address to check if it's alive, then optionally it is resolving hostname and tries to connect at specified in Options dialog box TCP port. The program uses separate threads for each scanned address to reduce scanning speed. It can also display NetBIOS information: computer name, workgroup, currently logged user and MAC address. No installer, just a small .exe file, great for quick scanning.
Download it from HERE (106kb)
GFI LANguard Network Security Scanner (NSS)
(currently v3.3) - LANguard allows you to not only scan your network, but do it from a Hackers perspective. It will identify all machines on your network, their Netbios info, open ports, shares and much more. LANguard Network Scanner can also display installed hotfixes and scan for known security issues - providing you with a web link to more information if any are found. It includes a large database of common vulnerabilities, including CGI, FTP, registry exploits and more. You can even customize the database, add custom criteria and specify alert flags based on your preference. With the information from LANguard security scanner, you can proactively starting securing your network - shutting down unnecessary ports, shares etc. The program also produces very nice HTML reports if needed. If you are interested in network security, you cannot afford not to use this tool! Even for non-professionals, it will provide great in depth information about your local LAN. (Some functions only available for NT/Win2k). Free version has some advanced features disabled (scheduled scans, report generator, result comparison deployment) but is well suited for most users.
See my GFI LANguard Network Security Scanner page for more info and download links.
(currently v99.2) - N.E.W.T. is a network scanner for administrators, that scans machines on a network and attempts to retrieve as much detailed information as possible without the need to run a client on the remote computer. It currently determines and lists Machine Name, IP Addresses, Operating System and Service Pack Info, Date and Time, Uptime, MAC Address, User Name, Domain Name and Machine Comments. Additional features include saving scan results as a text file for printing or archiving, sending popup messages to any or all remote NT machines, ping computers by IP address, reboot or shutdown remote machines with optional countdown and the ability to abort.
Download it from HERE (553kb).
(currently v220.127.116.11) - YAPS is short for "Yet Another Port Scanner", and this is exactly what it is. In fact, Yaps is a very basic but small and fast TCP/IP port scanner with little configuration options and a fairly plain interface. You can specify the IP range as well as the port range to scan and the program will display a list of all open ports found, including details.
Download it from HERE (72kb).
More will follow.