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To connect two IPv4 LANs, place a Linux machine on both networks, enable IPv4 forwarding on that machine, and add a route on the sending host that points to the router’s address on its own subnet. This lab uses 192.168.110.0/24 and 192.168.120.0/24; the commands are temporary and are intended for a controlled KVM, VirtualBox, or physical test network.
Why two subnets need a router
Hosts on the same IPv4 subnet can deliver frames through their local switch. A host sending to a different subnet cannot deliver that traffic directly: it sends the packet to a router, which forwards it through another interface and broadcast domain.
This example keeps the networks separate:
| Device or network | Address | Purpose |
|---|---|---|
| LAN 1 | 192.168.110.0/24 |
First broadcast domain |
| Router interface on LAN 1 | 192.168.110.126 |
Next hop for hosts on LAN 1 |
| Host 1 | 192.168.110.125 |
Originating host |
| LAN 2 | 192.168.120.0/24 |
Second broadcast domain |
| Router interface on LAN 2 | 192.168.120.136 |
Router address on LAN 2 |
| Host 2 | 192.168.120.135 |
Destination host |
The router must have one interface attached to each isolated network. Do not connect both virtual networks to the same bridge or switch, or you will no longer be testing routed traffic between separate LANs.
Choose a safe lab layout
KVM or VirtualBox
Create two virtual switches or bridges. Attach the router VM to both, Host 1 only to the 192.168.110.0/24 network, and Host 2 only to the 192.168.120.0/24 network. Interface names vary by distribution and hypervisor; the tutorial uses ens3 for Host 1, but your system may show names such as enp1s0 or another predictable name.
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A physical test needs three computers, two Ethernet switches, and Ethernet cabling. Connect the router computer to both switches, then connect each host to only its corresponding switch. An unmanaged Ethernet switch is sufficient for this basic two-LAN topology.
Inspect the Linux router first
On the router, confirm that both interfaces have addresses in the correct networks:
ip addr show
ip route showThe route table should contain connected routes for both 192.168.110.0/24 and 192.168.120.0/24. If an interface is down, attached to the wrong virtual network, or has an address from the wrong subnet, fix that before testing forwarding.
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Enable IPv4 forwarding
Check the current setting
sysctl net.ipv4.ip_forwardA value of 0 means the kernel is not forwarding IPv4 packets between interfaces. A value of 1 permits forwarding, subject to any firewall policy.
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echo 1 > /proc/sys/net/ipv4/ip_forwardThis is the tutorial’s temporary lab command. It is not persistent and is not a complete production router configuration. Firewall rules, filtering, logging, and an appropriate persistent configuration still need to be designed for a real network.
Add a static route on Host 1
Host 1 needs to know that the second network is reachable through the router interface on Host 1’s own LAN. Run this on Host 1:
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ip route add 192.168.120.0/24 via 192.168.110.126 dev ens3The via address is 192.168.110.126, not the router’s address on LAN 2. The dev value must be Host 1’s actual interface connected to LAN 1.
Verify the route:
ip route showYou should see a route for 192.168.120.0/24 using 192.168.110.126. In practical terms, Host 1 can now access that network through the router interface at 192.168.110.126.
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Routing is two-way. Host 2 must also know how to reach 192.168.110.0/24. You can provide that route explicitly on Host 2:
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ip route add 192.168.110.0/24 via 192.168.120.136Alternatively, configure the router interface on each LAN as the host’s default gateway. Use only one deliberate design: an explicit lab route is useful for learning, while a default gateway is common when the router is responsible for all off-subnet traffic.
Test in a useful order
- From Host 1, ping the router’s LAN 1 address,
192.168.110.126. - From Host 2, ping the router’s LAN 2 address,
192.168.120.136. - From Host 1, ping Host 2 at
192.168.120.135. - From Host 2, ping Host 1 at
192.168.110.125after installing the return route or setting the appropriate gateway. - On each machine, use
ip route showto confirm that the route selected for the destination points to the expected next hop.
A failed ping does not identify the fault by itself. Virtual-machine and distribution combinations can produce inconsistent ping results, so check interface state, addresses, routes, forwarding, and firewall behavior at each hop.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Remove the temporary route and reset the lab
On Host 1, remove the route with:
ip route del 192.168.120.0/24Remove the corresponding route from Host 2 if you added one. The tutorial’s route and forwarding changes are nonpersistent: they disappear after a restart. That makes them convenient for experiments but unsuitable as the final configuration for a server or permanent router.
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Persist the configuration on a current distribution
For a lasting setup, save addresses, gateways, forwarding policy, and static routes in the network-management system your distribution actually uses. Common choices include NetworkManager, systemd-networkd, and netplan; configuration file names and commands differ by release. After saving the configuration, reboot or restart the relevant network service and verify again with ip addr show, ip route show, and sysctl net.ipv4.ip_forward. Do not assume that a command which works interactively will survive a reboot.
Troubleshooting: why one subnet cannot ping the other
The router cannot reach both LANs
- Confirm each router interface is connected to the intended virtual bridge or physical switch.
- Check that the LAN 1 interface has an address in
192.168.110.0/24and the LAN 2 interface has an address in192.168.120.0/24. - Use
ip route showto verify both connected routes.
Forwarding is disabled
Run sysctl net.ipv4.ip_forward on the router. If it returns 0, enable forwarding for the lab and test again.
Host 1 has no route to LAN 2
Run ip route show on Host 1 and confirm the route specifies 192.168.120.0/24, next hop 192.168.110.126, and the interface attached to LAN 1.
The destination has no return route
Host 2 must send replies toward 192.168.120.136, either through an explicit route to 192.168.110.0/24 or through a suitable default gateway.
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A firewall blocks forwarded traffic
Successful pings to each router interface prove local connectivity, not that the router is allowed to forward packets. Review the router’s firewall and any host firewall rules before changing the network design.
Quick Recap
What this exercise teaches
- A subnet boundary is a routing boundary, not merely a different label.
- A Linux router needs interfaces in both networks and IPv4 forwarding enabled.
- The sender needs a route to the remote network, and the receiver needs a return path.
ip route showis often more informative than a single ping result.- Interactive
ipandsysctlchanges are excellent for a disposable lab but must be translated into distribution-specific persistent configuration for regular use.
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