Explanation
Network devices come with default credentials, the username and password that enables you to log into the device for configuration. Changing default credentials should be the first step in hardening a new device. As with any other system, avoiding common passwords adds security.
Keep network devices up to date. That means upgrading firmware, patching and updating as necessary to close any security gaps exposed over time.
Services on network devices like routers include common things, like Telnet and HTTP access; and also things you don't normally see, like TCP and UDP small services, debugging and testing tools that primarily use ports
20 and lower. If enabled, these services can be used to launch DoS and other attacks. All modern Cisco devices have these disabled; Cisco's hardening rules insist on disabling unnecessary services.
Using secure protocols hardens network devices. Don't use Telnet to access a managed switch or router, for example, but use SSH so that the communication is encrypted.
Disabling unused ports on network devices enhances port security for access control.
This includes standard IP ports and device ports, both physical and virtual for the latter.