As mentioned by other posters, there is the acceptance network and there is the card issuer.

Diners Club used to be an independent acceptance network, with independent banks issuing Diners Club cards, typically limited to one franchisee per country. This is similar to how there are independent banks issuing cards on the Visa and MasterCard networks.

However, the next part is where it starts to get muddled.

In the North America, the Diners Club acceptance network was disbanded years ago by Citi, and a deal was struck with MasterCard to use their network instead in the U.S. and Canada for Diners Club cards issued by Citi and the various Diners Club franchisees. Hence the MasterCard logos on North American cards, and the MasterCard logo on the back of cards issued overseas prior to the Discover purchase of the Diners Club network.

Overseas, however, many franchisees ran their country's own Diners Club acceptance network in addition to issuing the card and therefore would not strike similar deals with MasterCard, since that would basically close a portion of the franchisee's business if they did. So the Diners Club network continued to exist overseas, run by individual franchisees.

So you had this weird situation where Diners Club cards in the U.S. & Canada used the MasterCard network worldwide, but Diners Club cards issued overseas used the MasterCard network in North America only and still used the remaining Diners Club network overseas.

Then Discover comes in and buys the Diners Club network, and then integrates the Discover Network with the Diners Club and Pulse networks. Regardless of whether you call the new network the "Discover Network" or a combined Discover / Diners Club Network does not matter. Basically Diners Club is no longer an independent acceptance network.

All new Diners Club cards are now issued on the Discover Network (although may be locally branded as Diners Club), with the exception of the ones in North America where contracts are in place that keep them as MasterCard. When those contracts expire, I am sure both Discover Network and MasterCard will be fighting for those contracts.

So, in summary, cards issued in North America are MasterCards, and cards issued overseas are Discover Network cards.

The logo on the front or back of the card will tell you which network your card is on and whether you need to look for a MasterCard or Discover Network logo. If you see a Diners Club logo, it basically means it's the Discover Network branded as Diners Club.