3 Ethernet and Network Devices

Ethernet frames on a local link

Ethernet is a family of wired local-area networking technologies standardized as IEEE 802.3. It carries data in frames across a local link. An Ethernet frame contains information needed to deliver data on that link, and its payload commonly contains an IP packet.

A transmitting interface sends synchronization information before the frame. A basic, untagged frame is typically 64–1518 bytes, measured from the destination address through the Frame Check Sequence (FCS). VLAN tags and other Ethernet variants can change the frame size.

Fields in an Ethernet frame

A common Ethernet frame contains these fields:

• Destination MAC address: Identifies the intended receiver on the local link.

• Source MAC address: Identifies the sending interface.

• Type/Length: In Ethernet II, identifies the payload protocol, such as IPv4 or IPv6; in IEEE 802.3 framing, it indicates payload length.

• Data and padding: Carries the payload. Padding may be added to meet the minimum frame size.

• Frame Check Sequence (FCS): Provides an error-detection check so a receiver can detect a damaged frame. It detects errors but does not repair them.

MAC addressing and IP addressing

A Media Access Control (MAC) address is a link-layer address used to deliver frames within a local network segment. A common MAC address is 48 bits, written as six hexadecimal pairs, such as 02:1A:2B:3C:4D:5E. Network interfaces may use globally assigned or locally administered addresses, so a MAC address is not always a permanent, factory-set identifier.

Destination MAC addresses can identify one interface through unicast, a group through multicast, or all devices on the local broadcast domain through broadcast. An Ethernet broadcast uses the address FF:FF:FF:FF:FF:FF.

MAC and IP addresses serve different purposes. A MAC address delivers a frame across a local link, while an IP address identifies a destination across networks. When a host sends traffic to a different IP network, it sends the local frame to its default gateway’s MAC address; the router then forwards the IP packet toward its destination.

How switches forward frames

A bridge connects LAN segments and forwards frames using MAC addresses. An Ethernet switch is essentially a multiport bridge. As it receives frames, a switch learns by recording each source MAC address and the incoming port.

For a known destination, the switch uses this table to send a frame out the appropriate port. If the destination is unknown, or the frame is a broadcast, the switch floods it to the other relevant ports in that local network or VLAN.

For example, when Computer A sends a frame to Computer B on the same switched LAN, the switch learns A’s MAC address on A’s port. If the switch already knows B’s port, it forwards the frame there rather than sending it to every port.

How routers forward packets

A router connects IP networks and forwards packets according to their destination IP addresses and its routing information. This is a Layer 3 forwarding decision between networks. A Layer 2 switch, by contrast, forwards frames within a local network using MAC addresses.

When routing a packet over Ethernet, the router removes the incoming link-layer frame and sends the packet in a new frame on the outgoing link. The source and destination MAC addresses apply to that particular link and usually change at each routed hop. The packet’s IP addresses generally remain the end-to-end source and destination, though some router functions can modify them.

Access points and other network devices

A wireless access point (AP) lets Wi-Fi devices connect to a network. In a typical setup, it bridges wireless clients’ traffic to a wired LAN, forwarding traffic between wireless and Ethernet media. An AP provides wireless access; it does not necessarily route between IP networks.

Other common devices have specialized functions:

• A Network interface card (NIC) connects a device to Ethernet or Wi-Fi and sends and receives link-layer frames.

• A Repeater regenerates or retransmits signals to extend a connection. It does not make forwarding decisions based on MAC or IP addresses.

• A Hub is a legacy multiport repeater that repeats incoming bits to its other ports rather than selectively forwarding frames.

• A Modem adapts signals for a service-provider access medium. A modem alone does not necessarily provide routing or Wi-Fi.

• A Firewall applies traffic rules to allow, block, or inspect communications. It may be a separate appliance or integrated into a router or another device.

Many home-network products combine roles such as router, Ethernet switch, firewall, and wireless access point in one enclosure. The roles remain distinct even when the hardware is combined.