While MTU Probe can in some cases successfully ping your ISP to determine the MTU setting they are using, in most cases that is not the key issue. Although most routers on the Internet today are set to a maximum MTU of 1500, many people using a dial-up connection experience their best results with the 576 setting.
If you find you are unable to ping certain sites because they don’t accept pinging, don’t worry about it. Experimentation is still the best route to optimize your system.
For some good tips to get you started here, see the “TCP/IP Parameters Tweaking Tips” in the “Getting Started” section of the TweakMASTER Help File, accessible from the Help Menu in the TweakMASTER program.