CCNA Cisco Certified Network Associate Study Guide - FTP Server

26 Chapter 1 � Internetworking
Data This is a packet sent down to the Data Link layer from the Network
layer. The size can vary from 46–1500 bytes.
Frame Check Sequence (FCS) FCS is a field at the end of the frame that
is used to store the cyclic redundancy check (CRC).
Let’s take a look at some frames caught on our trusty Etherpeek network
analyzer. The frame below has only three fields: a destination, source, and
type field. This is an Ethernet_II frame. Notice the type field is IP, or 08-00
in hexadecimal.
Destination: 00:60:f5:00:1f:27
Source: 00:60:f5:00:1f:2c
Protocol Type:08-00 IP
The next frame has the same fields, so it must also be an Ethernet_II
frame. We included this one so you could see that the frame can carry more
than just IP: It can also carry IPX, or 81-37h. Notice that this frame was a
broadcast. You can tell because the destination hardware address is all 1s in
binary, or all Fs in hexadecimal.
Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 02:07:01:22:de:a4
Protocol Type:81-37 NetWare
Notice the length field in the next frame. This must be an 802.3 frame.
Which protocol is this going to be handed to at the Network layer? It doesn’t
specify in the frame, so it must be IPX. Why? Because when Novell created
the 802.3 frame type (before the IEEE did and called it 802.3 Raw), Novell
was pretty much the only LAN server out there. So, Novell was assuming
that if you’re running a LAN, it must be IPX.
Flags: 0x80 802.3
Status: 0x00
Packet Length:64
Timestamp: 12:45:45.192000 06/26/1998
Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 08:00:11:07:57:28
Length: 34