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Unreliable and best-effort do not mean that the system is unreliable and doesn’t work well,
but that the IP protocol does not make any effort to see if the packet was delivered. This function is
handled by the upper-layer protocols.

As information flows down the layers of the OSI model, the data is processed at each layer. At
the network layer, the data is encapsulated within packets called datagrams.

IP determines the form of the IP packet header (which includes addressing and other control
information) but does not concern itself with the actual data. It accepts whatever is passed down
from the higher layers, as shown in Figure 5-11.
https://www.youtube.com/watch?v=rLNmrFh-sd8&t=28s

Figure 5-11 shown an IP Header

5.9 Creating a Subnet

To create subnets, you must extend the routing portion of the address. The Internet

“knows” your network as a whole, identified by the Class A, B, or C address, which defines 8, 16, or
24 routing bits (the network number). The subnet field represents additional routing bits so that the
routers within your organization can recognize different locations, or subnets, within the whole
network. Subnet masks use the same format as IP addresses.

In other words, each subnet mask is 32 bits long and is divided into four octets. Subnet masks
have all 1s in the network and subnetwork portion and all 0s in the host portion. By default, if no bits
are borrowed, the subnet mask for a Class B network is 255.255.0.0.
However, if 8 bits were borrowed, the subnet mask for the same Class B network would be
255.255.255.0, as shown in Figure 5-12 and Figure 5-13. However, because there are two octets in
the host field of a Class B network, up to 14 bits can be borrowed to create subnetworks.

A Class C network has only one octet in the host field. Therefore, only up to 6 bits can be
borrowed in Class C networks to create subnetworks. The subnet field always immediately follows
the network number.

That is, the borrowed bits must be the first n bits of the default host field, where n is the
desired size of the new subnet field, as shown in Figure 5-14. The subnet mask is the tool used by the
router to determine which bits are routing bits and which bits are host bits.

5.9.1 Determining Subnet Mask Size

Again, subnet masks contain all 1s in the network bit positions (determined by the address
class) as well as the subnet bit positions, and they contain all 0s in the remaining bit positions,
designating them as the host portion of an address.