Legacy networks are not recommended. Many newer GCP features are not supported in legacy networks. Instead, use Virtual Private Cloud (VPC) networks. For more information, see the VPC Network Overview.
About legacy networks
Legacy networks have a single RFC 1918 range, which you specify when you create the network. The network is global in scope and spans all cloud regions.
In a legacy network, instance IP addresses are not grouped by region or zone. One IP address can appear in one region, and the following IP address can be in a different region. Any given range of IPs can be spread across all regions, and the IP addresses of instances created within a region are not necessarily contiguous.
The figure below shows a legacy (non-VPC) network. Traffic from the Internet passes through a global switching function in the network (shown in the diagram as a virtual switch), then down to individual instances.
Instances in a region can have IP addresses that are not grouped in any way.
As shown in the example, instances from 10.240.0.0/16 are spread unpredictably
across regions 1 and 2. For example,
10.240.1.4 is in region 2,
is in region 1, and
10.240.1.6 is in region 2.
It is not possible to create regional subnets with a legacy network.
Legacy networks start with only two routes, the default route to outside the network and the route to the overall legacy network IP range. See Using Routes for instructions on creating routes.
User-created networks have a default Allow-all firewall rule for outbound traffic and a default Deny-all firewall rule for inbound traffic. See Using firewall rules for instructions on creating firewall rules.