2 Network Devices and Their Roles

Distinct roles in a network

Networks use specialized devices to connect computers, move data, provide access, and enforce security. A single home device may combine several functions, while larger networks often use separate devices for each job. Identify a device by what it does rather than by its name or appearance.

Switching within a local network

A switch connects devices on the same local area network (LAN), such as computers, printers, and wireless access points. A typical Ethernet switch learns which MAC addresses are reachable through each port, then forwards a frame toward the port associated with its destination. Broadcast frames and frames with unknown destinations may be sent to multiple ports in the same network segment.

Managed switches can provide features such as virtual LANs (VLANs), traffic monitoring, and access controls. A Layer 3 switch can also route IP traffic between networks or VLANs.

Routing between IP networks

A router connects separate IP networks and forwards packets toward their destinations. It examines the destination IP address and uses a routing table to select an outgoing interface or next hop. A device typically sends traffic for other networks to its default gateway, usually a router on the local LAN.

For example, a home router forwards packets between the home network and the Internet. Routers can forward both IPv4 and IPv6 traffic. Many home routers also include a switch, Wi-Fi access point, firewall, and services such as DHCP and, commonly, IPv4 network address translation (NAT). Those are combined features, not the definition of routing itself.

Wireless and Internet access

A wireless access point (AP) lets Wi-Fi devices join a network. It bridges wireless clients onto a wired LAN, usually through an Ethernet connection to a switch. In an office, multiple APs may provide coverage across different areas while connecting clients to the same broader network. An AP provides wireless access; it does not necessarily route traffic between networks. A wireless router combines an AP with routing and often other functions.

A modem connects customer equipment to an Internet service provider’s access network by converting signals used on the service link into a form the customer’s network equipment can use, and vice versa. The technology depends on the connection, for example cable or DSL. With fiber service, an optical network terminal (ONT) commonly provides the corresponding connection at the customer’s premises. A modem or ONT provides the service link; a separate router may distribute the connection to multiple devices. Some equipment combines these functions.

Security and supporting devices

A firewall applies security rules to network traffic, allowing, blocking, or sometimes inspecting traffic according to policy. It may be a dedicated appliance, a feature in a router, or software on a computer or server. A firewall can help limit unwanted connections, but it does not replace secure configuration, updates, or endpoint protection.

Other devices support connectivity in different ways:

• A hub repeats incoming Ethernet signals to all its ports. It is older and mostly obsolete, and unlike a switch, it does not selectively forward frames to a particular port.

• A bridge connects or separates LAN segments and forwards frames based on link-layer information. Ethernet switches perform a similar function across multiple ports.

• A repeater or extender regenerates or retransmits a signal to extend a link or wireless coverage; it does not make routing decisions.

• A network interface card (NIC) is the wired or wireless interface that enables an end device to communicate with a network. Each network interface has a link-layer identity, such as a MAC address for Ethernet and Wi-Fi.

• A gateway is a general term for a device or service that connects different networks or systems. In everyday IP networking, the default gateway is usually the router a host uses to reach other networks.

How devices work together

A small office might connect computers and printers to a switch, connect that switch to a router, and connect one or more access points to the switch. The router’s WAN-side connection may lead to a modem or fiber ONT, which links the local network to the ISP. A firewall may be built into the router or placed separately at the network boundary.

The device used to forward traffic depends on where it is going. When a laptop sends data to a printer on the same LAN, a switch can forward the traffic locally. When the laptop sends data to a website on another network, it sends the traffic to its default gateway; the router then forwards it toward the ISP. A firewall may allow or block the connection according to its rules.