G

MAC Addresses

It is recommended that you use the default MAC address given by the factory, however, you may override that default with a MAC address of your own choosing via the Logicmaster 90 Module Configuration screen for the Ethernet module.

Each byte of the MAC Address is an 8-bit binary number. Thus, the 12-digit hex address is really a 48-bit binary number. The typical MAC Address shown above is represented as a binary number as follows:

Byte

 

1

 

2

 

3

 

4

 

5

 

6

 

________ _________ _________ _________ _________ _________

Hex

0

8

0

0

1

9

0

0

5

3

1

2

Binary

0000

1000

0000

0000

0001

1001

0000

0000

0101

0011

0001

0010

Another characteristic that is important, especially for multi-vendor networks, is the order of address-bit transmission on the physical medium. MAC Addresses are transmitted in ascending byte order, with the least significant bit of each byte transmitted first.

The example above is shown including bit transmission order as follows:

Byte

 

1

 

2

 

3

 

4

 

5

 

6

 

________

_________

_________

_________

_________

_________

Hex

0

8

0

0

1

9

0

0

5

3

1

2

Binary

0000

1000

0000

0000

0001

1001

0000

0000

0101

0011

0001

0010

Bit Order 8765

4321

 

...9

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSB of

the MAC

LSB of the MAC Address-first bit

transmitted

Address-last bit

 

 

 

 

 

 

 

 

 

transmitted

 

If you assign your own MAC Addresses, there are 2 bits of the 48-bit address that you must set according to the instructions that follow:

HBit 1 in Byte 1 must always be a 0 to indicate an individual station rather than a Group address.

HBit 2 in Byte 1 must be a 1 to indicate that the address is locally administered. (In the typical default MAC Address shown above, bit 2 in Byte 1 is a 0 indicating that it is a globally administered address).

HAll other bits can be set as desired as long as all stations on the network have unique addresses.

An example of a locally administered MAC Address is shown below.

Byte

 

1

 

2

 

3

 

4

 

5

 

6

 

________

_________

_________

_________

_________

_________

Hex

0

2

0

0

0

0

0

0

0

1

0

0

Binary

0000

0010

0000

0000

0000

0000

0000

0000

0000

0001

0000

0000

Bit Order 8765

4321

 

...9

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSB of the

LSB of the MAC Address-first bit transmitted

 

 

MAC Address-last

 

 

 

 

 

 

 

 

 

 

 

bit transmitted

If you must change the Station MAC Address, check with the person administering your network to make sure that you select an address that fits into your local administration scheme.

G-4

TCP/IP Ethernet Communications User's Manual ± January 1996

GFK-1004B

Page 248
Image 248
GE GFK-1004B user manual MAC Addresses