Time to mention some online, free diagnostic tools:

www.dnsstuff.com
Are people having trouble finding your web site, or is expected eMail not getting through? Use this site to verify your DNS mappings and Mail Server mappings.

www.broadbandreports.com
Are you getting all the bandwidth that you are paying for? Find out by running the speed tests from this site, and find out how your site is compared to others in your region, or from the same provider.
My current results:

Hmmmmm…

Shields Up!
Go to this site to see your current “public” IP address. The site will also check for open or closed ports on your firewall.

All three sites have tons of information explaining how the tests work, and the underlying internet plumbing.