Chapter 5


What is a MAC address?


Answer: A MAC address is the physical address of a network device and is 48 bits (6 bytes) long. MAC addresses are also known as the physical addresses and hardware addresses.


What are the components of a MAC address?


Answer: A MAC address is made up of two parts: the organizational unique identifier (OUI) and the vendor assigned serial number, or address.


How is a MAC address represented?


Answer: A MAC address is represented in hexadecimal format, specifically as six pairs of hexadecimal numbers separated by hyphens.


How does Ethernet operate?


Answer: Ethernet is a shared media LAN operating at 10-Mbps, 100-Mbps, 1000-Mbps (Gigabit) or 10000-Mbps (10 Gigabit) modes. 10/100 Ethernet and Gigabit Ethernet can operate in either half-duplex or full-duplex mode. 10 Gigabit Ethernet operates in full-duplex mode only. When in half-duplex mode, Ethernet operates in a CSMA/CD environment, whereas in full-duplex mode each device has dedicated access to network bandwidth.

Carrier sense multiple access collision detect (CSMA/CD) enables half-duplex Ethernet operation, in that each device listens to the line before sending its data. If the line is in use, meaning the device senses a carrier on the line, it holds off transmitting. If the line is not in use, meaning no carrier is sensed on the line, the device begins transmitting.


What is a collision, and what happens when a collision occurs on an Ethernet network segment?


Answer: A collision occurs when two or more devices attached to an Ethernet network segment send traffic across the line at the same time. When a collision occurs, each affected device waits a random amount of time before resending the data. The length of the wait is determined by a backoff algorithm.


Describe a repeater and how it operates.


Answer: A repeater repeats incoming signals from one interface and regenerates, or resends, the signal out all other interfaces.


Describe a hub and how it operates.


Answer: A hub is a central device in a network that joins lines together in a star configuration. Ethernet hubs are multiport repeaters often with 4, 8, 16, or 24 ports. A signal received on one port is repeated out all ports to all connected devices; it is up to each device to determine by the destination MAC address if it is the intended recipient of the transmission.


Describe a bridge and how it operates.


Answer: A bridge is a multiport hub and is used to connect two or more LAN segments together. A bridge uses a filtering table to determine whether a frame received from one LAN segment is to be filtered (dropped) or forwarded to another LAN segment connected to the bridge.


Describe a switch and how it operates.


Answer: A switch is a multiport bridge that cross connects two LAN nodes together, giving each sender/receiver pair the full network bandwidth rather than the shared bandwidth environment created by hubs. Switches separate collision domains by filtering and forwarding frames based on the source and destination hardware, or MAC, addresses. When a switch receives frames destined for an unknown address, the switch broadcasts these frames out all ports.


Describe a router and how it operates.


Answer: Routers are devices that forward data packets from one local-area network (LAN) or wide-area network (WAN) to another LAN or WAN. Routers forward these packets based on their routing tables, which are built using certain routing protocols. Routers read the network address from the packet within each transmitted frame and select a transmission method for the packet based on the best route available. Routers also drop packets destined for an unknown address. An unknown address is an address that is not found in the routing tables.


What is a backoff algorithm?


Answer: The backoff algorithm is a random amount of time each station waits after a collision has occurred on the network segment before attempting data retransmission.


What Ethernet technologies operate in half-duplex mode? Full-duplex mode?


Answer: Half-duplex: 10 Mbps (10BASE-T), 100 Mbps (100BASE-T), 1000 Mbps (Gigabit Ethernet). Full-duplex: 10 Mbps (10BASE-T), 100 Mbps (100BASE-T), 1000Mbps (Gigabit Ethernet), 10000 Mbps (10 Gigabit Ethernet). 10-Mbps and 100-Mbps Ethernet can operate in either half-or full-duplex mode depending on the network interface card and switch port capability.


Given the MAC address 00-aa-00-62-c6-09, identify the OUI and vendor-assigned serial number.


Answer: OUI: 00-aa-00

Serial number: 62-c6-09


How does a bridge determine whether a frame is forwarded or filtered?


Answer: Bridges determine whether to forward a frame based on the entries in its filtering table. If a frame is received from a port and the frame is destined for the network segment on that same port, the bridge filters (drops) the frame from its memory because the intended recipient is on the same network segment and would have received the frame. If the destination network segment is on a different bridge port, the bridge forwards the frame out that intended port.


What is the difference between cut-through and store-and-forward switching?


Answer: Cut-through switching forwards an incoming data frame before the frame is completely received by the switch. Store-and-forward switching stores the complete incoming frame before forwarding the frame on to its intended destination.


In what LAN environment are routers most commonly used?


Answer: In a local-area network (LAN) environment a router is commonly used to connect LANs together across a wide-area network (WAN) or to connect a LAN to the Internet.