12 Questions
What is the main purpose of the data link layer?
Encapsulate IP packets into frames
What is the role of MAC addresses in networking?
Identifying the sender and recipient of frames
What type of message is forwarded to a specific group of devices?
Multicast
Which statement accurately describes Ethernet addressing?
MAC addresses are 48-bit physical addresses
What do switches use to make forwarding decisions in local networks?
MAC addresses
When PC-A sends a frame to PC-B, what does the switch do?
Adds or updates a MAC address table entry for PC-A
What does a switch do if the destination address is not in the MAC address table?
Forwards the frame out of all ports except the incoming port
In which situation are broadcast and multicast frames handled similarly?
When they are sent out of all ports except the incoming port
What issue can arise when multiple switches are connected to each other?
Formation of layer-2 loops
Which traffic type on the data link layer can lead to collisions on a shared medium?
Half duplex
What does a VLAN allow you to do in a physical network?
Divide it into virtual subnets
Which standard is associated with wireless LAN connectivity?
IEEE 802.11
Study Notes
Data Link Layer
- Receives IP packets from the network layer and encapsulates them into frames.
- Uses MAC addresses to identify the sender and recipient of frames.
Ethernet
- Most common standard used in wired LAN networks.
- Ethernet frame consists of three parts: header, payload, and trailer.
MAC Addressing
- MAC addresses are 48-bit physical addresses used to identify host devices in a local network.
- Each network interface card (NIC) has a unique MAC address that does not change.
Ethernet Address Types
- Unicast messages: intended for a single, known recipient.
- Multicast messages: forwarded to a specific group of devices.
- Broadcast messages: forwarded to all devices in the local network segment.
Switches
- Layer-2 devices that forward frames in local networks based on MAC addresses.
- Store information about connected devices and their MAC addresses in a MAC address table.
- Fill the MAC address table based on the source addresses of incoming frames.
- Forward frames based on the MAC address table.
- If a destination address is in the MAC address table, the switch forwards the frame out of that port.
- If the destination address is not in the MAC address table, the switch forwards the frame out of all ports except the incoming port.
Loop Prevention
- Spanning tree protocols (STP) prevent layer-2 loops by logically blocking redundant paths.
- Loops can quickly bring down the entire network if not prevented.
Duplex
- Half duplex: only one end of a connection can send data at a time, while the other end listens.
- Full duplex: both ends of a connection can send data and listen at the same time.
- Modern wired networks use full duplex.
VLANs
- Virtual LANs (VLAN) divide a physical network into virtual subnets.
- Based on the IEEE 802.1q (dot1q) standard.
- VLANs are handled by switches that tag data with VLAN tags to specify what VLAN the data belongs to.
Wi-Fi
- IEEE 802.11 is a group of standards for wireless LAN connectivity.
- Latest Wi-Fi standard is Wi-Fi 6e (or 802.11ax), published in 2021.
Test your knowledge about the data link layer, the second layer in the OSI model. Learn about encapsulating IP packets into frames, MAC addresses, Ethernet standards, and the components of an Ethernet frame.
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