IOS
The primary Cisco operating system is IOS. There are some variations on the OS in different devices, but they generally work the same (I assume much like different versions of Linux).
Note that there are a few, newer versions of the OS - specifically NX-OS, IOS-XR, and IOS-XE. Although their CLIs are nearly identical, the main reason for a different version is that vanilla IOS runs as a monolithic kernel, which means if a single process crashes, the entire device can crash.
The newer OSs have microkernels, and processes run in separate, protected memory address spaces. As a result, crashing a process doesn’t necessarily mean crashing the entire device.
In Band / Out of band connections
In band connections are accomplished via the production network - the normal user traffic for staff members and the like.
There is often a separate, dedicated management network that is not to be used by normal staff members known as the out of band network. Used as a backup path for connections into a specific device.
Making initial connections to IOS
If a device does not have an IP address, we can begin by connecting via a console connection. This allows us to configure an IP address on the device itself and then access it over the network.
Console connections will normally be accomplished using a serial DB9 to RJ45 cable (a console cable). Note that often times our laptops will not have serial connectors; as a result, we will need a serial to USB connector.
Newer devices use USB to mini-USB.
Once we are connected at Layer 1
When our computer is connected to the management port of the device, we must establish a serial connection. If using PuTTY, we have to write the correct port (COM1, COM2, etc.) and the speed - on Windows we can see the communication ports available in Device Manager.
Once this is done, we can turn the device on and we will have access to the IOS command line.
This is all particularly userful for initial configs and also for troubleshooting.