endpoint within a local-area network (LAN). This section
introduces several networking concepts: Ethernet, MAC
addresses, VLANs, and switching.
Ethernet
A LAN is a network that is confined within a building or
campus. These types of networks start on the smaller
side with home networks, where end-user devices such
as personal computers, smartphones, laptops, and
printers connect—usually through a wireless connection
—to a local home network. The main characteristic of
these networks is that the data traffic between the
devices connected to the LAN stays local and can be
transferred between these devices by a switch, which is a
network device that operates at the data link layer and
enables direct communication between devices
connected to a LAN.
The most common LAN technology is Ethernet. Ethernet
is a data link layer protocol defined by the IEEE. The
Ethernet protocol encompasses guidelines and standards
that specify cabling and signaling formats at the physical
and data link layers. For example, Ethernet standards
specify characteristics for different types of cables and
network interface ports, and they also specify voltage
levels and the maximum distance the signal can travel
before it becomes too attenuated to be successfully
received. Over time, the Ethernet protocol has evolved
from transfer rates of 10 Mbps to 100 Gbps and even 400
Gbps. The transmission medium has also evolved from
coaxial cable to twisted-pair cable, fiber optics, and
wireless.
As mentioned previously, the PDU at the data link layer
is called a frame. The Ethernet protocol specifies the
format shown in Figure 16-5 for the Ethernet frame,
which includes the following fields: