Why Two Networks Connected by a Switch Cannot Communicate Directly

When connecting two networks using a switch, and each network contains two devices, there can be confusion about why devices in the first network cannot communicate with devices in the second network. This article explains the phenomenon and the role of a network switch and a router in network communication.

The Role of Switches in Network Communication

A switch is a device that allows multiple devices to communicate within the same network segment. Unlike consumer equipment, professional gear has more complex configurations, making it harder to provide a generalized answer. However, for our consumer-focused devices, a switch simply interconnects devices within the same network segment, allowing direct communication.

Before devices can communicate, the switch must “learn” where the devices are located on the network. This learning process may take several network cycles, sometimes a couple of days in some environments, to ensure that all devices are correctly identified.

Understanding Subnets and Network Segmentation

Two devices can only communicate directly if they are members of the same subnet. In IPv4, a subnet could be 192.168.1.0/24, with host addresses ranging from 192.168.1.1 to 192.168.1.254. Physical network segments can be as simple as a single Wi-Fi network or as complex as all devices interconnected by Ethernet cables. The original Ethernet standards, such as 10BASE5, connected all devices to the same physical cable.

An Ethernet switch is merely a device that allows for increased connectivity within the same logical network segment. However, this doesn’t exclude the possibility of running multiple subnets on the same physical or logical network segment.

Unable to Communicate Between Different Subnets

Let’s consider an example where we have two distinct subnets: the first subnet is 192.168.1.0/24 with addresses 192.168.1.1 and 192.168.1.2, and the second subnet is 192.168.253.0/24 with addresses 192.168.253.1 and 192.168.253.2. Here, devices 1 and 2 can communicate directly because they are in the same subnet. Similarly, devices 3 and 4 can communicate directly. However, devices 1 or 2 cannot communicate directly with devices 3 or 4 because they are in different subnets.

The Role of Routers in Network Segmentation

A single switch cannot route between subnets. To achieve communication between different subnets, a router must be used. A router can have an address on each subnet, such as 192.168.1.254 for the first subnet connected to devices 1 and 2, and 192.169.253.254 for the second subnet connected to devices 3 and 4.

When device 1 needs to communicate with device 4, it needs to determine whether the target device is in the same subnet. If they are in different subnets, device 1 sends its packets to the router, which then forwards the packets to the appropriate subnet. This process ensures that the correct subnet address is used for communication.

Configuring Subnets for Direct Communication

If you define the actual subnet as 192.168.0.0/16, it would cover devices with addresses 192.168.0.1 to 192.168.255.254. In this case, the switch can be used directly, and the devices can maintain their same addresses. However, the subnet mask definition in each device must be adjusted to reflect the correct subnet size, preventing unnecessary use of the router for direct communication within the same subnet.

Understanding the role of switches and routers in network communication and subnet management is crucial for ensuring effective and efficient network setup and operation. Whether you are a network administrator or a casual user, knowledge of these concepts can help you troubleshoot and design networks that meet your needs.

Keywords: network switching, subnet, router, network communication, network segmentation