This part of the checklist concerns routers, firewalls, and other network-level devices in a TCP/IP network, and their recommended configuration. One of the approaches to network security utilizes the Open Systems Interconnection (OSI) Reference Model used to describe network protocols and devices. In the OSI Model, everything in a network is divided into seven categories, called layers, from the physical layer up to the application layer, as follows:
Data link layer
The approach is to secure the entire network and its nodes from the bottom up. In other words, you start with the first layer, physical (securing equipment, cables, ...