It kind of depends on the test site you are using and your home network configuration. If you are using an external hardware based firewall such as the Cisco PIX 501 or any of the SOHO firewall products, your internal address space (typically 192.168.1.x) should be undetectable from an outside source. If the address is being detected, it could be a sign that NAT is not properly configured on your network, and that is a legitimate concern.
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If you are not using an external hardware based firewall and the IP address that is being reported is the address that Verizon is assigning you, that isn't of concern, since that address must be known on the Internet to allow for communications. I would recommend that, in addition to your current configuration, you also implement a dedicated hardware based firewall for the additional security and anonymity of your internal address space.
In regards to the test sites, the most popular site out there, www.grc.com can detect your internal IP address if you have installed a client "shim" from them. This essentially circumvents the NAT masking that occurs, allowing GRC to report on your actual internal address. If you have installed this client component, then there is nothing to worry about, since it is working as it should.
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