Ethernet Frames
Written on 04:00 by Ahmet Emir
The Abstracts Link band is amenable for accumulation $.25 into bytes and bytes into frames. Frames
are acclimated at the Abstracts Link band to abbreviate packets handed bottomward from the Arrangement layer
for manual on a blazon of media access.
The action of Ethernet stations is to canyon abstracts frames amid anniversary added application a group
of $.25 accepted as a MAC anatomy format. This provides absurdity apprehension from a circadian redundancy
check (CRC). But remember—this is absurdity detection, not absurdity correction. The 802.3 frames
and Ethernet anatomy are apparent in Figure 1.20.
Encapsulating a anatomy aural a altered blazon of anatomy is alleged tunneling.
FIGURE 1 . 2 0 802.3 and Ethernet anatomy formats
Preamble
8 bytes
Preamble
8 bytes
DA
6 bytes
SA
6 bytes
Length
2 bytes Abstracts FCS
DA
6 bytes
SA
6 bytes
Type
2 bytes Abstracts FCS
4 bytes
Ethernet_II
802.3_Ethernet
36 Chapter 1 Internetworking
Following are the capacity of the altered fields in the 802.3 and Ethernet anatomy types:
Preamble An alternating 1,0 arrangement provides a 5MHz alarm at the alpha of anniversary packet,
which allows the accepting accessories to lock the admission bit stream.
Start Anatomy Delimiter (SFD)/Synch The conference is seven octets and the SFD is one octet
(synch). The SFD is 10101011, area the aftermost brace of 1s allows the receiver to appear into the
alternating 1,0 arrangement about in the average and still accompany up and ascertain the alpha of
the data.
Destination Abode (DA) This transmits a 48-bit amount application the atomic cogent bit
(LSB) first. The DA is acclimated by accepting stations to actuate whether an admission packet
is addressed to a accurate node. The destination abode can be an alone abode or a
broadcast or multicast MAC address. Remember that a advertisement is all 1s (or Fs in hex) and
is beatific to all accessories but a multicast is beatific alone to a agnate subset of nodes on a network.
Source Abode (SA) The SA is a 48-bit MAC abode acclimated to analyze the transmitting
device, and it uses the LSB first. Advertisement and multicast abode formats are actionable within
the SA field.
Length or Blazon 802.3 uses a Length field, but the Ethernet anatomy uses a Blazon acreage to analyze the
Network band protocol. 802.3 cannot analyze the upper-layer agreement and charge be acclimated with a
proprietary LAN—IPX, for example.
Data This is a packet beatific bottomward to the Abstracts Link band from the Arrangement layer. The admeasurement can
vary from 64 to 1,500 bytes.
Frame Check Sequence (FCS) FCS is a acreage at the end of the anatomy that’s acclimated to store
the CRC.
Let’s abeyance actuality for a minute and booty a attending at some frames bent on our accurate OmniPeek
network analyzer. You can see that the anatomy beneath has alone three fields: Destination, Source,
and Blazon (shown as Agreement Blazon on this analyzer):
Destination: 00:60:f5:00:1f:27
Source: 00:60:f5:00:1f:2c
Protocol Type: 08-00 IP
This is an Ethernet_II frame. Apprehension that the blazon acreage is IP, or 08-00 (mostly aloof referred to
as 0x800) in hexadecimal.
The abutting anatomy has the aforementioned fields, so it charge be an Ethernet_II anatomy too:
Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 02:07:01:22:de:a4
Protocol Type: 08-00 IP
Did you apprehension that this anatomy was a broadcast? You can acquaint because the destination hardware
address is all 1s in binary, or all Fs in hexadecimal.
Let’s booty a attending at one added Ethernet_II frame. I’ll allocution about this abutting archetype afresh when
we use IPv6 in Chapter 13, but you can see that the Ethernet anatomy is the aforementioned Ethernet_II frame
Ethernet Networking 37
we use with the IPv4 baffled agreement but the blazon acreage has 0x86dd back we are accustomed IPv6 data,
and back we accept IPv4 data, we use 0x0800 in the agreement field:
Destination: IPv6-Neighbor-Discovery_00:01:00:03 (33:33:00:01:00:03)
Source: Aopen_3e:7f:dd (00:01:80:3e:7f:dd)
Type: IPv6 (0x86dd)
This is the adorableness of the Ethernet_II frame. Because of the agreement field, we can run any
Network band baffled agreement and it will backpack the abstracts because it can analyze the Network
layer protocol.
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