Network switches work differently from one another. Some just move data from one device to another. Others make smarter decisions based on the type of traffic moving through. A Layer 4 switch does more than pass data, as it understands more about what that data is doing.
It pays attention to information from the TCP/IP protocol layers, especially the transport layer of the OSI model. In this article, we’ll break it all down: what Layer 4 of the OSI model means, how these switches work, and why they matter in everyday networks.
What Is a Layer 4?
OSI layer 4 is called the transport layer. It's part of both the OSI reference model and the TCP/IP model. The job of this layer is to make sure data gets from one place to another the right way. It handles things like error checks and how fast the data goes. If something fails, this layer can fix it or ask for it again.
Protocols like TCP and UDP live at this layer. TCP checks that data gets through safely, while UDP just sends it and moves on. If you've streamed a video or sent an email, the TCP/IP layers did the work.
The transport layer in the OSI model also makes it possible for many services to run at once on the same device. That’s because each service uses a port number, which tells the system what kind of traffic it is, like web, email, or gaming.
Now, when a switch understands these port numbers and protocols, it's working at Layer 4. That’s what makes a Layer 4 switch different.
How Does a Layer 4 Switch Work?
A regular switch looks at MAC addresses. A Layer 3 switch can check IP addresses. But a Layer 4 switch? It reads transport protocol layer info. This includes the TCP layer and UDP headers. That way, it can tell what type of service the data is for.
For example, a Layer 4 switch can spot traffic meant for a video call and treat it differently from an email. It can move that traffic faster, slower, or even send it through a backup line.
It’s all about smarter traffic handling. The TCP/IP in the OSI model allows this kind of control by defining clear layers and what they do. Layer 4 switches use that structure to make real-time choices.
What Can You Do with a Layer 4 Switch?
Layer 4 switches help when networks get busy or services need attention. They’re used in places where speed matters more than, such as video quality, voice clarity, or real-time gaming. Furthermore, they help with:
- Load balancing: spreading traffic across multiple servers
- Traffic prioritization: putting video or voice first
- Security: filtering based on ports or known threats
- Managing services: giving rules for specific types of traffic
These switches make decisions based on layer 4 protocols, like HTTP (port 80), HTTPS (port 443), or DNS (port 53). So the switch doesn't just see where data is going, but it knows why it's going there.
Where Does Layer 4 Fit in the Models?
Both the OSI model and TCP stack have similar layers, though they group them differently. In the OSI model layer 4, you get reliable delivery. The TCP/IP layer 4 does the same thing, but as part of a smaller model with fewer layers.
The layers below it are Layer 3 and Layer 2, which deal with routing and local delivery. The TCP/IP model OSI link connects them all. The Layer 3 OSI model uses IP addresses, and Layer 2 works with MAC addresses. Together with Layer 4, these form the logic that routers and switches use.
The layers in the TCP/IP model and the layers in the TCP IP protocol may sound confusing. But they boil down to:
- Application
- Transport (TCP IP layer 4)
- Internet
- Network Access
When a switch works at Layer 4, it interacts directly with the TCP / UDP protocols.
Key Differences Between Layer 2, 3, and 4 Switches
The Open Systems Interconnection (OSI) model defines how network devices communicate. Switches operate at different layers of this model, offering varying functionalities based on the information they can process in a data packet. Here's a comprehensive table outlining the key differences between Layer 2, Layer 3, and Layer 4 switches:
Feature |
Layer 2 (Data Link Layer) Switch |
Layer 3 (Network Layer) Switch |
Layer 4 (Transport Layer) Switch |
OSI Layer |
Layer 2 (Data Link Layer) |
Layer 3 (Network Layer) |
Layer 4 (Transport Layer) |
Addressing Used |
MAC Addresses (Physical Addresses) |
IP Addresses (Logical Addresses) |
TCP/UDP Port Numbers (Application-specific ports) |
Primary Function |
Switching: Forwards data frames within the same LAN/VLAN based on MAC addresses. |
Routing: Forwards data packets between different LANs/VLANs (subnets) based on IP addresses. Also performs switching. |
Application-aware traffic management & load balancing: Forwards packets based on application port numbers and can perform more intelligent traffic manipulation. |
Data Unit |
Frame |
Packet |
Packet |
Broadcast Domains |
Creates one broadcast domain per VLAN. Cannot break up broadcast domains on its own. |
Can break up broadcast domains (by routing between VLANs/subnets). |
It can help manage and filter traffic within broadcast domains, but doesn't primarily define them. |
Collision Domains |
Each port is its own collision domain. |
Each port is its own collision domain. |
Each port is its own collision domain. |
VLAN Support |
Supports VLANs for logical segmentation within a single broadcast domain. |
Supports VLANs and can route traffic between VLANs (Inter-VLAN routing). |
Can understand VLANs for more granular traffic control, especially for applications. |
Routing Protocols |
No (only switching). |
Yes (e.g., RIP, OSPF, EIGRP, Static Routes). |
Typically integrates with routing protocols from Layer 3, but its core function is not routing itself. |
Performance |
Very fast (hardware-based forwarding). |
Fast (hardware-based routing, often using ASICs, faster than traditional routers for inter-VLAN routing). |
Can be slower than L2/L3 for raw forwarding due to deeper packet inspection, but optimizes application performance. |
Complexity |
Simple to configure and manage. |
More complex than L2 switches due to routing functionalities. |
Most complex due to application-level awareness and advanced features. |
Typical Use Case |
Small to medium-sized LANs, workgroup connectivity, access layer of a network. |
Core or distribution layer of larger networks, inter-VLAN routing, campus networks, data centers. |
Server load balancing, content switching, traffic prioritization (QoS) for specific applications, enhanced security (e.g., firewall-like capabilities). |
Cost |
Least expensive. |
More expensive than L2 switches, less expensive than dedicated routers for internal routing. |
Most expensive due to specialized hardware and software. |
WAN Connectivity |
No (cannot connect to wide area networks). |
No (typically lacks WAN ports, relies on routers for WAN connectivity). |
No direct WAN connectivity. |
Intelligence |
"Dumb" in terms of IP addresses; only understands MAC addresses. |
"Smart" enough to understand IP addresses and routing tables. |
"Intelligent" to understand applications and services based on port numbers. |
Examples of Features |
MAC address learning, VLANs, Spanning Tree Protocol (STP), and port security. |
Inter-VLAN routing, static/dynamic routing protocols, ACLs (Access Control Lists) based on IP. |
Load balancing, traffic shaping, Quality of Service (QoS) based on application, and advanced security policies (e.g., filtering by port). |
Why Do Layer 4 Switches Matter?
More devices mean more apps, which will lead to more traffic. Today’s networks need smart ways to keep data moving. Layer 4 switches help by looking deeper than just addresses. They look at services, types of traffic, and how urgent they are.
In busy places like schools, offices, or data centers, these switches keep things running smoothly. No bottlenecks. No stuck services. Just smart traffic control based on the transportation layer in the OSI model. You might see this switch used in setups where:
- Multiple services run at once
- Video or voice needs to stay clear
- Web apps need load balancing
- Firewalls or filters need port-based rules
These real uses show how understanding the TCP IP structure can improve network design.
Takeaway
A Layer 4 switch is a step up. It doesn’t just pass along packets. Instead, it understands them and uses info from the transport layer of the OSI model to make better decisions. This helps busy networks stay fast and stable.
TS Cables offers switches, accessories, and cables that work well with OSI model examples, from simple Layer 2 setups to advanced Layer 4 protocols. Browse our network solutions built to handle real performance, not just promises.
FAQs
1. What layer is TCP in the OSI model?
TCP works at OSI Layer 4, which is called the transport layer.
2. What is the main job of the transport layer?
It handles reliable delivery, error checks, and flow control using TCP or UDP.
3. What layer is UDP in?
UDP also operates at Layer 4 of the OSI model, like TCP, but it's connectionless and faster.
4. What's the difference between Layer 3 and Layer 4 switches?
Layer 3 switches route by IP. Layer 4 switches make decisions based on port numbers and service types.
5. Why would I use a Layer 4 switch?
To control traffic types, support services like voice or video, and apply smart rules based on transport protocol layer data.