Angry IP Scanner

Angry IP scanner is a very fast IP scanner and port scanner. It can scan IP addresses in any range as well as any their ports. Its binary file size is very small compared to other IP or port scanners. Angry IP scanner simply pings each IP address to check if it's alive, then optionally it is resolving its hostname, determines the MAC address, scans ports, etc. The amount of gathered data about each host can be extended with the available plugins.

It also has additional features, like NetBIOS information (computer name, workgroup name, and currently logged in Windows user), favorite IP address ranges, customizable openers, etc.

Scanning results can be saved to CSV, TXT, HTML, XML or IP-Port list file, can be used as a command-line utility in a batch file, etc. With help of plugins, Angry IP Scanner can gather any information about scanned IPs. Anybody who can write code is able to write plugins and extend functionality of Angry IP Scanner.

In order to increase scanning speed, it uses multithreaded approach: a separate threads is created for each scanned IP address.

It is free and open-source software, so use it at your own risk.

Disclaimer: The author is not responsible for anything you do using the program.

This program is mostly useful for network administrators to monitor and manage their networks.
For more information about IP and port scanning in general, you can see the corresponding articles on Wikipedia or Wikiverse.

tcptraceroute

DAG: tcptraceroute RPM packages for Red Hat/Fedora
tcptraceroute is a traceroute implementation using TCP packets.

The more traditional traceroute sends out either UDP or ICMP ECHO packets with a TTL of one, and increments the TTL until the destination has been reached. By printing the gateways that generate ICMP time exceeded messages along the way, it is able to determine the path packets are taking to reach the destination.

The problem is that with the widespread use of firewalls on the modern Internet, many of the packets that traceroute sends out end up being filtered, making it impossible to completely trace the path to the destination. However, in many cases, these firewalls will permit inbound TCP packets to specific ports that hosts sitting behind the firewall are listening for connections on. By sending out TCP SYN packets instead of UDP or ICMP ECHO packets, tcptraceroute is able to bypass the most common firewall filters.