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A netmask is a 32-bit binary mask used to divide an IP address into subnets and specify the network's available hosts.

Netmask primarily provides a method to create small subnetworks from a large range of IP addresses. Generally, netmask length is defined in up to 24-bit format for all types of IP classes. The division or creation of networks into subnetworks depends on the class of IP address in use along with their available netmasks. For example, the netmasks for the three IP classes are:

255.0.0.0 for Class A with an 8-bit netmask
255.255.0.0 for Class B with a 16-bit netmask
255.255.255.0 for Class C with a 24-bit netmask

The greater the length of netmask the more networks it can accommodate. Therefore, the number of hosts decreases from Class A to Class C, whereas the number of available networks or subnetworks increases.

In a netmask, two of the possible addresses, represented as the final byte, are always pre-assigned and unavailable for custom assignment. For example, in 255.255.225.0, "0" is the assigned network address. In 255.255.255.255, the final "255" is the assigned broadcast address. These two values cannot be used for IP address assignment.

Below is an example of a netmask and an example of its binary conversion.

Netmask:	255.	255.	255.	255
Binary:	11111111	11111111	11111111	11111111
Netmask length	8	16	24	32

Counting out the bits in the binary conversion allows you to determine the netmask length. Above is an example of a 32-bit address. However, this address is a broadcast address and does not allow any hosts (computers or other network devices) to be connected to it.
A commonly used netmask is a 24-bit netmask, as seen below.

Netmask:	255.	255.	255.	0
Binary:	11111111	11111111	11111111	00000000
Netmask length	8	16	24	--

Using a 24-bit netmask, the network would be capable of 2,097,150 networks or 254 different hosts with an IP range of 192.0.1.x to 223.255.254.x, which is usually more than enough addresses for one network.
A simple formula can be used to determine the capable amount of networks a netmask can support.

2^^{(netmask length - # of used segments)} - 2

For example, if we used a netmask length of 24, having a netmask of 255.255.255.0 with three used segments, subtract three from the netmask length, e.g., 24-3 = 21. With this number determined, plug it into the above formula to get 2^²¹ - 2 = 2,097,150 total number of networks. You are subtracting two from this number because of the broadcast and network addresses that are already being used.
Another example is a netmask length of 16, having a netmask of 255.255.0.0 with two used segments. Using the above formula, you would get 2^¹⁴ - 2 = 16,382 total number of networks.
To determine the number of hosts a netmask is capable of supporting, use the following formula.

2^^{(# of zeroes)} - 2

For example, with a netmask length of 24, as shown in the above chart, there are eight zeroes. Therefore, using the formula above, this would be 2^⁸ - 2 = 254 total number of hosts. Again, two is subtracted from this number to account for the broadcast and network addresses.
Again, another example of a netmask length of 16, there would be 16 zeroes. The formula, in this case, would be 2^¹⁶ - 2 = 65,534 total number of hosts.
Below is a breakdown of each of the commonly used network classes.

Class	Netmask length	# of networks	# of hosts	Netmask
Class A	8	126	16,777,214	255.0.0.0
Class B	16	16,382	65,534	255.255.0.0
Class C	24	2,097,150	254	255.255.255.0

Mounting Filesystems in Linux

On Linux and UNIX operating systems you can use the mount command to attach (mount) file systems and removable devices such as USB flash drives at a particular mount point in the directory tree.
The umount command detaches (unmounts) the mounted file system from the directory tree.
In this tutorial, we will go over the basics of attaching and detaching various file systems using the mount and umount commands.

For more linuxize.com/post/how-to-mount-and-unmount-file-systems-in-linux

Routing Tables

A routing table is a set of rules, often viewed in table format, that is used to determine where data packets traveling over an Internet Protocol (IP) network will be directed. All IP-enabled devices, including routers and switches, use routing tables.
A routing table contains the information necessary to forward a packet along the best path toward its destination. Each packet contains information about its origin and destination. When a packet is received, a network device examines the packet and matches it to the routing table entry providing the best match for its destination. The table then provides the device with instructions for sending the packet to the next hop on its route across the network.
A basic routing table includes the following information:

Destination: The IP address of the packet's final destination
Next hop: The IP address to which the packet is forwarded
Interface: The outgoing network interface the device should use when forwarding the packet to the next hop or final destination
Metric: Assigns a cost to each available route so that the most cost-effective path can be chosen
Routes: Includes directly-attached subnets, indirect subnets that are not attached to the device but can be accessed through one or more hops, and default routes to use for certain types of traffic or when information is lacking. Routing tables can be maintained manually or dynamically. Tables for static network devices do not change unless a network administrator manually changes them. In dynamic routing, devices build and maintain their routing tables automatically by using routing protocols to exchange information about the surrounding network topology. Dynamic routing tables allow devices to "listen" to the network and respond to occurrences like device failures and network congestion.

Example:
source: https://searchnetworking.techtarget.com/definition/routing-table