The national computing curriculum in Czechia was revised in 2021 to incorporate various topics, including fundamental principles of the internet and computer networks at the ISCED level 2 (age 11–15). However, schools face a shortage of age-appropriate teach- ing materials on this subject. To address this gap, we developed four model lessons and corresponding materials tailored for ISCED 2 students and teachers at two levels of difficulty. The primary objective of these lessons is to elucidate the structure and funda- mental principles of the internet, building on prevalent student preconceptions. This work is grounded in the constructivist instruc- tional framework known as Evocation – Realisation of Meaning – Reflection (ERR). The lessons were created and validated through design-based research conducted across six schools, involving 13 classes and a total of approximately 270 students. Pre-post-delayed testing revealed very large immediate improvements following the intervention (d = 1.72, p < 0.001, n = 204) as well as large, sustained effects months later (d = 1.10, p < 0.001, n = 59). This paper details the development, implementation, and evaluation of these lessons, providing a valuable resource for educators in various contexts.