Notes

DNS, or the Domain Name System, translates human readable domain names (for example, www.amazon.com) to machine readable IP addresses (for example, 192.0.2.44).
Linux mount and umount. The mount command mounts a storage device or filesystem, making it accessible and attaching it to an existing directory structure. The umount command "unmounts" a mounted filesystem, informing the system to complete any pending read or write operations, and safely detaching it.
NFS (Network File System) is basically developed for sharing of files and folders between Linux/Unix systems by Sun Microsystems in 1980. It allows you to mount your local file systems over a network and remote hosts to interact with them as they are mounted locally on the same system.

Service Port
SMTP 25
DNS 53
FTP 20 (data transfer) , 21 ( Connection established)
DHCP 67/UDP(dhcp server) , 68/UDP(dhcp client)
SSH 22
Squid 3128


under Linux the boot loader is called Grub or LILO


A run level is a state of init and the whole system that defines what system services are operating. Run levels are identified by numbers. Some system administrators use run levels to define which subsystems are working, e.g., whether X is running, whether the network is operational, and so on.

Whenever a LINUX system boots, firstly the init process is started which is actually responsible for running other start scripts which mainly involves initialization of you hardware, bringing up the network, starting the graphical interface.
Now, the init first finds the default runlevel of the system so that it could run the start scripts corresponding to the default run level.
A runlevel can simply be thought of as the state your system enters like if a system is in a single-user mode it will have a runlevel 1 while if the system is in a multi-user mode it will have a runlevel 5.
A runlevel in other words can be defined as a preset single digit integer for defining the operating state of your LINUX or UNIX-based operating system. Each runlevel designates a different system configuration and allows access to different combination of processes.
The important thing to note here is that there are differences in the runlevels according to the operating system. The standard LINUX kernel supports these seven different runlevels :

0 – System halt i.e the system can be safely powered off with no activity.
1 – Single user mode.
2 – Multiple user mode with no NFS(network file system).
3 – Multiple user mode under the command line interface and not under the graphical user interface.
4 – User-definable.
5 – Multiple user mode under GUI (graphical user interface) and this is the standard runlevel for most of the LINUX based systems.
6 – Reboot which is used to restart the system.
By default most of the LINUX based system boots to runlevel 3 or runlevel 5.
In addition to the standard runlevels, users can modify the preset runlevels or even create new ones according to the requirement. Runlevels 2 and 4 are used for user defined runlevels and runlevel 0 and 6 are used for halting and rebooting the system.