Lumen will retain its copper mass-market network. Why it matters: Lumen's ballooning debt has become burdensome, preventing it from adequately investing in its consumer fiber network business.

AT&T agreed to buy Lumen Technologies' Mass Markets fiber internet connectivity business for $5.75 billion in cash, sending shares of the Denver-based firm soaring in premarket trading Thursday.

AT&T will acquire substantially all of Lumen's Mass Markets fiber business, which today totals about 1 million fiber customers and reaches more than 4 million fiber locations across 11 U.S. states.