2 Link-Layer Networking and Ethernet

Physical and data-link functions

Network communication divides work across layers. The Physical layer sends bits as signals over a transmission medium, while the Data-link layer organizes bits into frames for delivery across one local link or LAN.

The physical-layer specification defines details such as signaling, connectors, and supported transmission rates. The medium may be copper cable, optical fiber, or a radio channel. At the data-link layer, link-level addressing and frame-boundary handling support local delivery, and transmission errors are commonly detected.

A frame delivers data to the next link-layer destination; it does not by itself provide delivery across different IP networks. Communication between different IP networks requires a router, which forwards traffic between links.

Ethernet and MAC addressing

Ethernet is a widely used family of wired LAN technologies standardized by IEEE 802.3. Ethernet data-link functions are described in IEEE 802.3, while IEEE 802.1 specifies bridging and VLAN behavior.

An Ethernet interface uses a MAC address to identify a link-layer source or destination. A MAC address is typically written as six hexadecimal octets, for example 00:1A:2B:3C:4D:5E.

Ethernet supports three destination patterns:

• Unicast: a frame is directed to one destination.

• Broadcast: a frame is directed to all devices in the local broadcast domain. The destination is commonly written FF:FF:FF:FF:FF:FF.

• Multicast: a frame is directed to a group.

Ethernet frame structure

An Ethernet frame is the link-level container for data. It carries destination and source MAC addresses, a length/type field, payload and possibly padding, and a Frame Check Sequence (FCS). The FCS lets the receiver check for certain transmission errors, but does not correct them.

The frame fields serve these purposes:

• Destination MAC address: identifies the intended link-layer recipient.

• Source MAC address: identifies the sending interface.

• Length/type: indicates the payload length or, in common Ethernet II frames, the payload protocol type.

• Payload and padding: carries upper-layer data; padding may be added when needed to meet the minimum frame size.

• Frame Check Sequence (FCS): provides an error-detection check.

A basic Ethernet MAC frame is conventionally described as 64 to 1518 bytes, counted from the destination address through the FCS. The preamble and start-frame delimiter are transmitted on the medium but are not included in that count. An IEEE 802.1Q VLAN tag adds four bytes, so a tagged frame may be 1522 bytes under this conventional count. Implementations may support larger frames as well.

For example, when a computer sends an IP packet in Ethernet, it places the packet inside the frame’s payload and uses destination and source MAC addresses in the frame header. The Ethernet frame and the IP packet it carries are distinct.

LAN switching and frame forwarding

A local area network (LAN) connects devices in a limited local area, such as a home, office, or campus network. Modern wired LANs commonly connect devices to an Ethernet switch.

A switch learns which MAC addresses are reachable through which ports by examining the source address of incoming frames. It records those address-to-port associations in a forwarding table. When a frame arrives, it checks the destination MAC address:

• If the destination is known, the switch forwards the frame toward the associated port.

• If the destination is unknown, the switch generally floods the frame to other ports in the same VLAN, except the port on which it arrived.

• Broadcast frames are forwarded within the same VLAN, but are not normally forwarded into other VLANs.

For example, when Computer A sends a frame to Computer B, the switch learns A’s MAC address and incoming port. If the switch already knows B’s port, it forwards the frame only there; otherwise, it floods the frame within the VLAN. When B replies, the switch learns B’s location, making later delivery more targeted.

Unlike a basic hub, which repeats incoming signals to its other ports, a switch makes forwarding decisions using frame addresses. Switched Ethernet links commonly operate in full-duplex mode, allowing sending and receiving at the same time. Original shared-medium Ethernet used CSMA/CD to handle collisions; collision handling is generally not needed on modern full-duplex switched links.

VLANs and broadcast domains

A Virtual LAN (VLAN) divides a switched physical network into separate logical LANs. Devices in different VLANs are in different broadcast domains even when they connect to the same physical switch. VLANs help organize network segments, but communication between them requires routing.

IEEE 802.1Q specifies VLAN-aware bridging and the use of VLAN tags to identify VLAN traffic on links that carry multiple VLANs.