4 Ethernet and Local Area Networks
Learn how Ethernet frames and MAC addresses carry data across a LAN, how switches forward traffic, and how link speed and duplex affect communication.
on a LAN
is a family of wired local-area networking technologies standardized as IEEE 802.3. In a typical LAN, computers, printers, and other devices connect to an . carries data in frames, and switches use the frames’ MAC addresses to deliver them across the local network.
What an carries
An packages data for transmission over an link. A common frame has these fields:
Destination : Identifies the intended local recipient or recipients.
Source : Identifies the sending interface.
Type/Length: Identifies the encapsulated protocol, such as IPv4 or IPv6, or gives the data length.
Payload: Carries the network-layer packet and its data.
: Provides an error-checking value so a receiver can detect frame corruption.
The preamble and start-frame delimiter help the receiver synchronize with an incoming transmission; they are commonly shown alongside the frame fields in diagrams. The FCS detects errors but does not repair them.
A frame might carry an IPv6 packet as its payload. delivers the frame across the LAN, while the IP packet inside it contains the network-layer information used to communicate between IP addresses.
MAC addresses and destination types
A is a 48-bit link-layer address, usually written as six pairs of hexadecimal digits, such as 00:1A:2B:3C:4D:5E. It identifies an interface for communication on the local link. Unlike an IP address, a is not used by routers to forward traffic between IP networks.
destinations can be , , or . The is FF:FF:FF:FF:FF:FF. A forwards frames within the relevant LAN segment or rather than using the destination MAC to send them to one specific device.
How switches forward frames
A builds a by examining the source of each incoming frame and recording the port on which that address was learned. When a frame arrives, it checks the destination and acts according to what it knows:
If the destination is known, the forwards the frame through the associated port.
If the destination is unknown, the floods the frame out other ports in the same so the destination can receive it. The continues learning from frames it receives.
frames are also flooded within the same .
A does not ordinarily send a frame back out the port on which it arrived. These forwarding decisions let devices on the same LAN exchange frames without sending every frame to every device.
For example, Computer A sends a frame to Computer B. The learns A’s from the incoming frame. If it already knows which port leads to B, it forwards the frame only through that port. If B’s address is not yet in its table, the floods the frame within the . B can reply, allowing the to learn B’s location too.
Link and
is the link’s signaling rate, expressed in bits per second. link speeds include , , , and higher rates. The actual link depends on the capabilities of both ends and the connection between them.
describes whether a link can transmit and receive at the same time:
: Both ends can send and receive simultaneously. This is the normal mode for a device connected directly to a port, and the link does not have collisions.
: Devices take turns transmitting. Collisions can occur on a shared medium, so traditional half- uses (Carrier Sense Multiple Access with Collision Detection) to detect collisions and retry transmissions.
Compatible ports commonly use to agree on supported and settings. A mismatch, such as one end using while the other uses , can cause poor performance and errors.
Communication within and beyond the LAN
When two devices on the same LAN communicate, the sender places the recipient’s in the and sends it to the . The uses its to forward the frame toward the correct port.
If the destination is outside the local IP network, the sender instead addresses the to the of its next-hop router. The router then forwards the IP packet toward the other network.