Cisco mac address access control mac#

The MAC control field contains all information usedįor flow control, connection establishment and teardown as well as error control. However most will have at a minimum the following fields: Since there are various types of Network Interfaces ( Ethernet, Token Ring,įDDI etc.) the MAC frame format differs by protocol according to it'sĭesign. Duplicate MAC addresses separated by one or more routers is not a problem since the two devices won't see each other and will use the router to communicate. Duplicate MAC addresses on the same LAN are a problem. On an Ethernet LAN, this will cause a high number of collisions. If two devices have the same MAC address (which occurs more often than network administrators would like), neither computer can communicate properly. No other device on that local network subnet can use that MAC address. In order for a network device to be able to communicate, the MAC address it is using must be unique. See ARP and RARP on how IP addresses are tranlated into MAC addresses and vice versa. MAC addresses are 'burned' into the Network Interface Card (NIC), and cannot be changed. Manufacturers re-use MAC addresses and they ship cards with duplicate addresses to different parts of the United States or the World so that there is only a very small chance two computers with network cards with the same MAC address will end up on the same network. Since there are more than 16.7 million computers in the world, this clearly isn't enough addresses for every computer on earth, is it? Duplicate MAC Addresses The 24 bits used to identify a host allows for up to 16.7 million unique card addresses on one network.

Cisco mac address access control series#

In the case of the protocols specified in the IEEE's 802.x series of documents, the first 24 bits of a MAC address identify the vendor-manufacturer of the network interface card and the last 24 bits identify the card itself, or more precisely, the last 24 bits identifies the specific host the network inteface card is attached to. Of 24 bits of the MAC address is assigned to a specific vendor as the OrganizationallyĬisco was assigned the hexadecimal prefix '00000C' to use on their first Manufacturers of network interface adaptor cards 'burn' a MAC address into Different companies like to show MAC addresses different ways.

The hexadecimal digits are the same, but they are separated or grouped differently when displayed. There is some difference in how they are displayed on different types of computers. Here are some examples of what a MAC address looks like. When looking at a hexadecimal representation of the MAC address, the first six hexadecimal digits identify the vendor and the last six hexadecimal digits identify the specific network interface card. The first 12- bit portion of the MAC address identifies the vendor of the network device, the last 12- bit portion identifies the unique id of the device itself. The MAC address uniquely identifies a specific network device and MAC addresses must be unique on a given LAN (a network of computing devices in a single subnet of IP addresses). The MAC address is most frequently represented as 12 hexadecimal digits. The IEEE 802.3 Ethernet and 802.5 Token Ring protocols specify that the MAC sub-layer must supply a 48- bit (6 byte) address. The MAC sublayerĪlso communicates with the Logical Link Control ( LLC) sub-layerĪbove it allowing it to access and speak to the upper layer network protocols The MAC sublayer provides the means to access the the physical medium used for communication. That control the electricity transmitted and received on a copper wire are Or radio frequencies) over the physical media.

That handle communication with the physical media (copper wire, fiber opticĬable or the air) by controlling the communication signal (electricity, light On every network interface adaptor card there The Media Access Control is often said to be a sub-layer of the OSIĭata Link layer.