EarthLink Network will begin offering high-speed Internet access nationwide through an agreement with UUNet Technologies, the companies are expected to announce today. EarthLink and UUNet, a unit of MCI WorldCom, have been testing the digital subscriber line, or DSL, service in Los Angeles and Boston and will roll it out to more cities by the end of the year, according to the companies. Pasadena-based EarthLink said the agreement makes it the first Internet service provider to offer high-speed DSL connections across the country. The DSL connections will cost between $40 and $60 a month and allow users to download data from the Internet about seven times faster than is possible with today's standard dial-up modems, an EarthLink spokesman said. EarthLink is also planning to roll out DSL service in Southern California later this month through a partnership with Pacific Bell. In Nasdaq trading, EarthLink shares dropped $6.88--or 10%--to close at $60.88, while MCI WorldCom shares gained $1.31 to close at $90.75.