Specifications

Ethernet Data Frames
4-10 Ethernet Data Frames
This frame type adds a five-byte protocol identification field at the end of
the 802.2 header, where the protocol is identified. To distinguish an IEEE
802.2 SNAP frame, the value of the DSAP and SSAP fields in the 802.2
header are both set to AA. If a network device finds AA in the DSAP and
SSAP fields, it knows this is a SNAP-based frame and it should look for
the protocol type in the protocol identification field.
Ethernet Addressing Schemes
There are three types of addressing schemes used in Ethernet networks.
Each address type serves a different purpose. They are as follows:
1. Specific Addressing
2. Multicast Addressing
3. Broadcast Addressing
Specific Addressing
The IEEE specifies that each addressable network device will have a
unique hardware address that is made up of 6 bytes of information. The
address is either hard-coded into every network interface controller card
during manufacturing or assigned out of a group of addresses given to a
particular corporation. The availability of addresses is strictly controlled
by the IEEE.
The IEEE assigns each network hardware manufacturer a unique
manufacturer identifier and a block of numbers that the manufacturer
usually assigns sequentially to each piece of hardware. The combination
of the manufacturer ID and the sequential number makes up the common
48-bit Ethernet address.
The first 3 bytes of the address contains the manufacturer identifier and
the last 3 bytes contain the sequential numbering. The numbering scheme
is given in hexadecimal format. An example of a typical Ethernet address
is shown in Figure 4-4.
Figure 4-4. A Typical Ethernet Address
00-00-1D-00-26-A3
Manufacturer ID Sequential Address
1913-08