Voting by Felons

If I recall correctly, this is, believe it or not, a state by state thing.

The constitution is mute on the subject--however, the 15th, 19th and 26th amendments expand the franchise, respectively prohibiting racial disqualification, guaranteeing the vote to women and guaranteeing the vote to those at least 18 years of age.

but people with felonies are not allowed to vote in majority of states .. no.

Yes, actually, they are permitted to vote in a large majority of states. Broadly speaking, About a quarter forbid felons voting in any circumstances. the rest allow them to vote while in prison, or after release , or after completion of parole or probation. And a couple don't restrict voting. There really are 50 different laws on the subject.
http://felonvoting.procon.org/view.resource.php?resourceID=286

Short answer: no it doesn't. States who want to allow felons to vote can do so under the US constitution.

Background: The US constitution of 1787 left the issue of voting rights completely up to the states. States could grant or deny the right to vote whichever way they saw fit.

Then a series of amendments subsequently reduced the state's power to deny individuals the right to vote, as Setanta said already. Currently, the constitution forbids states to deny the vote on account of race (15th Amendment), sex (19th Amendment) failure to pay a poll tax (24th Amendment), and any age equal or greater than 18. (26th Amendment). States can still deny the right to vote for other reasons, including your criminal record.

But they don't have to. No amendment to the US constitution ever constrained the states' right to allow people to vote. This brings us back to the short answer: No, the US constitution doesn't impinge on states allowing felons to vote, because it doesn't impinge on them allowing anyone to vote.

Shouldn't that be 51? What about voting rights in U. S. territories, such as Puerto Rico, Guam, American Samoa, etc.?