Software-based network isolation provided by routers is not as efficient as believed, as hackers can smuggle data between the networks for exfiltration.
Most modern routers offer the possibility to split the network into multiple segments that work separately. One example is a guest network that works in parallel with the host.
The boundary insulates sensitive or critical systems from others that enjoy less strict security policies. This practice is common and even a recommended security measure. It is a logical separation that occurs at software level, though, and it is not airtight.
Researchers at the Ben-Gurion University of the Negev discovered multiple methods to carry data across two segregated network segments on the same hardware.
Source: Bleeping Computer