Simple definition
A router sends IP packets from one network to another. It is often the default gateway used by devices to reach other subnets or the Internet.
Technical definition
A router examines each packet's destination IP address and selects the best matching route, normally by longest-prefix match. Routes can be configured statically or learned from routing protocols.
- Client device 192.168.10.0/24 subnet
- Default gateway Subnet router
- Routing table Longest prefix match
- Next hop Interface or router
- Destination Other IP network
Routing uses the destination IP address and the best matching route. Firewall policies may still restrict traffic.
What is it used for ?
Connect subnets, VLANs, remote sites and the Internet, while using security policies to control which traffic may pass.
Practical example
A workstation in VLAN 10 reaches a server in VLAN 20 through a router or Layer 3 switch. The router uses its routing table; firewall policies may restrict the permitted connections.
Common issues
- Missing route to the destination subnet
- Asymmetric routing or incorrect return route
- Routing loop between devices
- Default gateway or next hop unreachable
Related terms
Key takeaway: Routing selects a network path; it does not automatically authorize access. Verify the destination route, return route and applicable security policy.