Some problems with network connections are hard to diagnose on a computer, as one can appear connected even when the IP address is invalid for DHCP connectivity, such as an APIPA. Testing such a connection can take some configuration checks which sometimes consumes a lot of time. Sentinel is a handy portable application for Network Administrators which can help troubleshoot such problems easily. It allows you to test your TCP communication to help determine if your computer is online. It uses TCP (Transmission Control Protocol) to send data in the form of message unit to help you check if the message is successfully sent.
Once you launch, Sentinel, you can enter an IP address (e.g. of a DHCP server) and click Connect to connect to a host or specify a server port and click Start. Once done, you will be able to send a test message.
If your computer is properly connected to the network, a new window will pop-up informing you that the message has been successfully sent.
Sentinel is an open source application which works on Windows XP, Windows Vista and Windows 7.