Crosssubnet

Crosssubnet refers to the ability of devices located on different IP subnets to communicate within a network. Since subnets segment address space and separate broadcast domains, inter-subnet communication requires a Layer 3 device that can route packets between subnets based on their destination addresses.

In practice, cross-subnet connectivity relies on routing. Each subnet is associated with a gateway or a Layer 3 interface on a router or multilayer switch. Hosts use the gateway as their default route, and the device forwards traffic to the appropriate next hop. Inter-subnet routing can be static, with explicit routes, or dynamic, using routing protocols such as OSPF, EIGRP, RIP, or BGP that distribute reachability information. IPv4 and IPv6 subnets are defined by masks or prefixes, and subnetting controls scale and isolation.

In modern networks, cross-subnet connectivity is often implemented within VLANs and data center fabrics, and may utilize overlay technologies (for example VXLAN, GRE) to carry traffic across an underlay network. Cloud environments may connect subnets across regions or VPCs via peering, VPN, or transit networks, enabling cross-subnet communication between isolated address spaces.

Security and policy considerations are important: firewalls, access-control lists, and micro-segmentation are commonly applied to regulate inter-subnet traffic. Network design best practices emphasize route summarization, minimum necessary routing complexity, and consistent addressing schemes.

See also: inter-VLAN routing, routing protocols, subnetting, NAT.