Gateway

A gateway is the crossing point between a local network and everything outside it — another subnet or the internet. Every device knows the address of its default gateway and sends any traffic there whose destination isn't inside its own subnet. This role is usually played by a router or a firewall at the edge of the network.

In practice, every device gets its gateway address either automatically through DHCP or entered manually, typically the first or last address in a subnet, such as 192.168.10.1. When a PC on the office subnet needs to reach a server on the internet, it first checks whether the destination is inside its own subnet; if not, the request is automatically forwarded to the gateway. Larger networks with several subnets can have multiple internal gateways, one per area.

A typical mistake is a wrongly entered gateway address: the device then talks to other devices on the same subnet without any trouble, but has no connection to anything outside it — a symptom that's easy to mistake for a general network outage. Similarly, a gateway that goes down cuts off an entire subnet from the rest of the infrastructure, even though internal traffic keeps working fine.