Figure 10 802.3 Frame W/ Snap Header - Control Technology Ethernet TCP/IP Adapter Module CTI 2572 Technical Overview

The Ethernet Version 2 standard defines the packet as follows:
Ethernet
Destination Address
6 bytes
The type field is used to define the upper level protocol being used (e.g. TCP/IP, DEC LAT,
etc.).
The IEEE 802.3 specification substitutes a length field for the type field and has the following
format:
Ethernet
Destination Address
6 bytes
Even though the Ethernet II and 802.3 packet formats are different, there are generally accepted
conventions for placing information in the Type/Length field which allows communications
software to differentiate between the two. As a result, both packet types can co-exist on the same
network. However, a node which sends and receives only Ethernet II packets cannot
communicate with a node which sends and receives only 802.3 packets.
Since IEEE 802.3 does not contain any information which identifies the upper level protocol
being used, it is usually combined with the 802.2 Logical Link Control (LLC) specification. The
LLC header does provide information about the upper layer protocol and is placed in the first few
bytes of the 802.3 data area.
However, the 802.2 specification
allows only one byte for the upper
level protocol identifier (called a
Service Address Point). Thus, a
special version of 802.2 LLC
called SNAP (Sub Network
Access Protocol) was created to
contain the 2 byte TYPE field
used in the Ethernet II packet.
14
Ethernet
Source Address Type
6 bytes 2 bytes
Ethernet
Source Address Length
6 bytes 2 bytes
6 Bytes 6 Bytes
Destination Source
Address Address
1 Byte 1 Byte
Destination Source
SAP SAP
170 = SNAP 170 = SNAP

Figure 10 802.3 Frame w/ SNAP Header

Data
Variable
Data
Variable
2 Bytes Variable (up to 1500 Bytes)
Data
Data
Length
IEEE 802.2
LLC Header
1 Byte 3 Bytes
LLC Organization
Control Code
03 = UI = 000
CTI 2572 Technical Overview Manual
CRC
4 bytes
CRC
4 bytes
4 Bytes
CRC
Data
2 Bytes
EtherType
0800 = IP