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A default gateway in Internet jargon is a term for a hardware node or point that will provide outgoing access to data packets to a destination in some other discrete network.
Default simply means that this gateway is used by default, unless an application specifies another gateway. The default server does not even need to be a router; it may be a computer with two network adapters, where one is connected to the local subnet and the other is connected to an outside network.
Experts might often explain that the default gateway uses the Internet protocol to act as a forwarding host for packets related to outbound communications.
The word “default” is important here, in that the default gateway is just the gateway that is used if no other gateway is specified.
It's common practice to have a gateway in a network structure to send data outside of that discrete network itself. In some cases, the default gateway will be replaced by a specification, but largely, the gateway itself is often a “default gateway” as a matter of course.
Part of understanding how a default gateway or other network gateway works is understanding the OSI model that governs modern networking.
The OSI model is composed of seven layers, starting with the underlying physical hardware layer. The next layer is the data link layer, or layer 2, that handles internal network communications. Layer 3 is the networking layer that handles communications between networks.
The network gateway operates at layer 3 of the OSI model. Like other types of communication between different networks, this falls into the networking layer category.
Successive layers of the OSI model include the transport layer or layer 4, the session layer or layer 5, the presentation layer or layer 6 and the application layer or layer 7. Each of these had its particular use and designation f components, that contribute to network admin.
In a traditional physical network, the default gateway is going to point to the IP address of piece of hardware that acts as the gateway. This can create some confusion.
Most commonly, the IP address specified is the IP address for the router. The router acts as the gateway, but the router, inherently, is just a piece of hardware that sends data. The router itself isn't inherently labeled as “the gateway” – it acts as the default gateway because of how the system is set up. So when people ask: “is the default gateway the router?” – in a way, it is, and in a way, it isn't.
In virtual networks, this becomes very different. In a virtual machine subnet, the default gateway is not going to be a piece of hardware, but an internal IP address signaling the allocation of virtual resources. For example, a virtual machine may act as a default gateway.
Then there's the software-defined wide area network or SD-WAN where modernization automates a lot of what used to be done manually. If you look at a flowchart for an SD-WAN, there's often no need to confirm a default gateway, because that process is already set up in network automation.
The above shows how a default gateway acts in a standard Internet networking environment. It also shows how the term is mostly a designation more than a description of a given piece of hardware in all systems.