A MAC address is a 48 bit number. The vendor code is the first 24 bits, and the last 24 bits represent a vendor assigned number. Although the 48 bit MAC addresses are written in hexadecimal, there are many ways to represent these addresses. Some examples representations are:
Hexadecimal and Converting Between Binary and Hexadecimal
The hexadecimal system, commonly called “hex”, uses 16 symbols (as compared to the two symbols used in binary or the ten symbols used in decimal). Since we represent these symbols with only one character, and there are only 10 Arabic numbers (0 through 9), we have to use 6 new symbols to represent these numbers. We therefore borrow from the alphabet, where:
Decimal 10 becomes Hexadecimal A
Decimal 11 becomes Hexadecimal B
Decimal 12 becomes Hexadecimal C
Decimal 13 becomes Hexadecimal D
Decimal 14 becomes Hexadecimal E
Decimal 15 becomes Hexadecimal F
(The letters A through F can be upper or lower case, or a mixture.)
Each hexadecimal digit can be represented by exactly 4 bits:
To convert a hexadecimal number to binary, convert each hexadecimal symbol to its binary equivalent. For example, the hexadecimal number:
af34 = 1 0 1 0 1 1 1 1 0 0 1 1 0 1 0 0
\ / \ / \ / \ /
a f 3 4
In Cisco routers, hexadecimal numbers are normally indicated by “0x” preceding the number. The above example number would therefore appear as 0xaf34.
Ethernet Frame Types
Field lengths are in bytes: SOF = Start of Frame; FCS = Frame Check Sequence
|Preamble||Dest MAC||Source MAC||Type||DATA||FCS|
Type field value shows type of protocol being carried, values 0x05DD – 0xFFFF
|Preamble||SOF||Dest MAC||Source MAC||Length||802.2 Header + DATA||FCS|
Length field value shows length of packet, values 0x0001 – 0x05DC (1 – 1500 bytes)
IEEE 802.2 LLC
|1||1||1 or 2|
|Destionation Service Access Point (DSAP)||Source Service Access Point (SSAP)||Control|
DSAP: Defines the communications pathway to the next level (network) protocol on the receiving side.
SSAP: Defines the communications pathway from the next level (network) protocol on the sending side.
Control: Defines the type of transmission.
IEEE 802.2 LLC_SNAP
|DSAP (0xAA)||SSAP (0xAA)||Control (0x03)||OUI||Protocol|
DSAP: Fixed at 0xAA.
SSAP: Fixed at 0xAA.
Control: Fixed at 0x03.
OUI: Organizationally Unique Identifier – manufacturers code from the MAC address.
Protocol: Type field from the original Ethernet_II frame type.
|Dest MAC||Source MAC||TPID||TCI||Length or Type||DATA||CRC|
TPID (Type Identifier) – 0X8100
TCI (Tag Control Information)
– 3 bits for priority
– 1 bit for Token Ring Encapsulation Flag
– 12 bits for VLAN ID
Unshielded twisted pair (UTP) cables for Ethernet can be either:
- Straight-through cables for connecting “dis-similar” devices (such as router-switch, switch-PC or server, hub-PC or server). Straight-through cables have each wire on one end connected to the same pin on the other end.
- Crossover cables for connecting “similar” devices (such as switch-switch, hub-hub, router-router, PC-PC, but also router-PC and switch-hub). Crossover cables have pin 1 on one end connected to pin 3 on the other end, and pin 2 on one end connected to pin 6 on the other end.
Next week we’ll go over WAN.
Excerpted and available for download from Global Knowledge White Paper: CCNA v1.1 Exam Review: Critical Concepts of the 640 – 802 CCNA Exam