CCNA Cisco Certified Network Associate Study Guide - FTP Server

Ethernet Networking 23
Ethernet is supposed to offer 100 percent efficiency in both directions. This
means that you can get 20Mbps with a 10Mbps Ethernet running full
duplex, or 200Mbps for FastEthernet. This is called an aggregate rate, but it
essentially means “you’re supposed to get” 100 percent efficiency, though no
one is certain.
When a full-duplex Ethernet port is powered on, it connects to the remote
end and then negotiates with the other end of the FastEthernet link. This is
called an auto-detect mechanism. This mechanism first decides on the
exchange capability, which means it checks to see if it can run at 10 or
100Mbps. It then checks to see if it can run full duplex. If it cannot, then it
will run half duplex.
Ethernet at the Data Link Layer
Ethernet at the Data Link layer is responsible for Ethernet addressing, which
is typically called hardware addressing or MAC addressing. Ethernet is also
responsible for framing packets received from the Network layer and preparing
them for transmission on the local network through the Ethernet contention
media access method. There are four different types of Ethernet
frames available:
� Ethernet_II
� IEEE 802.3
� IEEE 802.2
� SNAP
I will discuss all four of the available Ethernet frames in the upcoming
sections.
Ethernet Addressing
Ethernet addressing uses the Media Access Control (MAC) address burned
into each and every Ethernet Network Interface Card (NIC). The MAC
address, sometimes referred to as a hardware address, is a 48-bit address
written in a canonical format to ensure that addresses are at least written in
the same format, even if different LAN technologies are used.
Figure 1.10 shows the 48-bit MAC addresses and how the bits are
divided.