Key Points
- Network bridges operate at Layer 2 of the OSI model and forward traffic based on learned MAC address tables.
- Three main bridge types exist: transparent, source routing, and translational, each suited to different network environments.
- Bridges differ from switches in that a switch is a multi-port bridge optimized for speed and volume.
- Security risks include ARP poisoning and MAC spoofing, both of which exploit the bridge’s inherent trust of MAC addresses.
- 802.1X certificate-based authentication is the recommended control for securing network bridge access.
A network bridge is a computer networking device that creates a single, aggregate network from multiple communication networks or network segments, operating at the data link layer (Layer 2) of the OSI model. Understanding how network bridges work, where they fit alongside switches and routers, and how to secure them helps IT teams make smarter infrastructure decisions.
What Is a Network Bridge?
A network bridge is a device or application that reduces network congestion by connecting and filtering traffic across network segments. Network bridges treat segments as a single network and forward Ethernet frames between segments based on learned MAC address tables.
Network bridges operate at Layer 2 (the data link layer) of the Open Systems Interconnection (OSI) 7 Layer model. A network bridge monitors all traffic passing through it and dynamically builds a MAC address table by observing source MAC addresses on incoming frames. That table maps Media Access Control (MAC) addresses to specific network ports.
A bridge determines where to forward each Ethernet frame by reading the source and destination MAC addresses within the frame. If the destination MAC address is associated with the same interface on which the frame was received, the bridge will filter the frame from passing through other segments. If both devices live on different segments, the network bridge will forward the frame directly to that segment.
How Does a Network Bridge Work?
A network bridge monitors all traffic passing through it and builds a MAC address table by observing source MAC addresses on incoming frames. When a frame arrives, the bridge reads its destination MAC address and checks its table.
If the destination is on the same segment as the source, the bridge drops the frame and prevents unnecessary traffic from crossing segments. If the destination is on a different segment, the bridge forwards the frame to the correct port.
This forwarding process is called transparent bridging and happens at wire speed. The bridge learns new MAC addresses automatically by noting which port each frame arrives on.
Over time, the MAC address table becomes a complete map of which devices live on which segment. When the bridge encounters a destination MAC address it has not yet learned, it floods the frame to all ports except the one it arrived on, ensuring the frame reaches its destination while the bridge continues to learn.
Types of Network Bridges
Not all network bridges work the same way. The three primary types reflect different network environments and use cases.
- Transparent bridges are the most common type in modern networks. They learn MAC addresses automatically by observing traffic. They also require no configuration from the network devices they serve, and they operate invisibly from the perspective of end devices. Most Ethernet bridges deployed today are transparent bridges.
- Source routing bridges are associated with Token Ring networks, which were more prevalent before Ethernet became dominant. In source routing, the sending device includes the full path through the network inside the frame itself, rather than letting the bridge determine the route. This type is rare in contemporary networks but may still appear in legacy environments.
- Translational bridges connect network segments that use different Layer 2 technologies, for example an Ethernet segment and a Wi-Fi segment. The bridge translates frames from one format to the other, handling differences in addressing, frame structure, and maximum transmission unit (MTU) size. Wireless access points that bridge between a wired Ethernet backbone and a Wi-Fi network are a practical example of translational bridging.
What Are the Benefits of Using a Network Bridge?
Network bridges help IT teams manage and optimize their local area networks (LANs). They also allow devices from separate networks to communicate as if they were on the same network.
Additional benefits include ease of local network expansion, reduced network congestion, better network performance, flexibility to use different LAN technologies, and minimal demands on the IT team.
Do Organizations Still Use Network Bridges?
Network bridges still perform a valuable function today for many organizations. But in recent years, switches and routers with more sophisticated functionality have emerged.
Ethernet switches have become far more popular than network bridges. Technically, an Ethernet switch is a multi-port bridge implemented in hardware for higher performance. Switches can handle many connections simultaneously and make faster forwarding decisions. But bridges remain a cost-effective choice for simpler network scenarios.
Why Use a Network Bridge Instead of a Switch?
Switches can handle many connections simultaneously and make faster forwarding decisions.
For high-traffic environments with many devices, a switch is the better choice. For smaller, simpler networks where cost is a constraint, a standalone bridge can still do the job without the overhead of a full managed switch.
Why Use a Network Bridge Instead of a Router?
Routers use IP addresses to direct packets of information across different networks. They also provide better traffic isolation than bridges. But if you’re simply extending a network rather than connecting separate ones, a bridge would be an appropriate choice.
What Is a Bridge Connection? What Is an Internet Bridge?
A bridge connection turns a Windows PC into a network bridge. A wireless bridge connects two network segments over Wi-Fi at Layer 2, often used for building-to-building connectivity.
How to Set Up a Network Bridge
Most IT teams set up network bridges on Linux servers. Here’s a rough outline of the process:
- Identify all available network interfaces using the ip link show command.
- Install the bridge-utils package.
- Configure your bridge by editing your network configuration files.
- Apply your changes by restarting your networking service.
- Assign an IP address to your new bridge interface.
Both Windows and macOS provide built-in support for creating network bridges.
What Are the Potential Security Risks of Using a Network Bridge?
Like other Layer 2 devices, network bridges are highly trusting. Bridges operate at Layer 2 and do not perform Layer 3 or application-level inspection, limiting their ability to enforce access control policies.
As a result, criminals target bridges in Address Resolution Protocol (ARP) poisoning attacks and use MAC spoofing to trick bridges into giving them access to segments of the network that contain sensitive data.
With limited filtering ability, network bridges let broadcast traffic travel into all the network segments they connect. This means broadcast storms and ARP-based attacks can travel far on the network.
MAC Flooding
MAC flooding is a common attack vector. By sending large volumes of frames with spoofed source MAC addresses, an attacker can overflow the bridge’s MAC address table, forcing it into a fail-open mode where it floods all traffic to all ports. At that point, the bridge behaves like a hub, and any device on the network can capture traffic intended for other segments.
Because bridges rely entirely on MAC addresses for access decisions, and MAC addresses can be cloned or spoofed in software, trusting MAC addresses alone is not a sufficient security model for any network that handles sensitive data.
How to Secure Your Network Bridge
Allowing your network bridges to trust MAC addresses will leave your network vulnerable to cyberattacks. Instead of doing that, require every device to authenticate using 802.1X port-based network access control with certificate-based authentication.
Certificate-based authentication removes the reliance on MAC addresses entirely. When a device presents a valid certificate, the network can verify its identity against a trusted certificate authority before granting access to any segment. This means a spoofed MAC address alone is not enough for an attacker to gain entry.
SecureW2 provides the infrastructure to deploy certificate-based authentication at scale. JoinNow Cloud RADIUS handles RADIUS authentication for both wired and wireless network access, while Dynamic PKI automates certificate issuance and renewal so devices always have a valid credential. Together, they replace MAC-based trust with a cryptographically verified identity model that network bridges and switches can enforce through 802.1X .
To see how certificate-based authentication works in your environment, schedule a demo with the SecureW2 team.
Frequently Asked Questions
What is a network bridge?
A network bridge is a computer networking device that creates a single, aggregate network from multiple communication networks or network segments. It operates at Layer 2 (the data link layer) of the OSI model and forwards Ethernet frames between segments based on a learned MAC address table.
What is the difference between a network bridge and a switch?
A switch is a multi-port network bridge implemented in hardware for higher performance. Both devices operate at Layer 2 and forward traffic based on MAC addresses. The practical difference is scale and speed: switches handle many simultaneous connections and are optimized for throughput, while traditional bridges are simpler devices suited to smaller or less complex network segments.
Are network bridges still used?
Yes, network bridges are still used in specific scenarios, particularly for extending smaller LANs, connecting building-to-building over Wi-Fi, and in embedded or IoT environments where a full managed switch is not practical. That said, managed Ethernet switches have replaced bridges in most enterprise network deployments.
How do you secure a network bridge?
The most effective way to secure a network bridge is to replace MAC address trust with 802.1X port-based network access control and certificate-based credentials. This ensures only devices with a valid, trusted certificate can connect to the network, regardless of what MAC address they present.
