LPIC-1 Essential System Services and Networking Practice Question
A Linux administrator needs to configure a system to act as a router between two subnets. The system has two network interfaces: eth0 (192.168.1.1/24) and eth1 (10.0.0.1/24). Which TWO steps are required to enable routing between these subnets? (Choose two.)
⚠ Common exam trap
The trap here is overlooking the need for firewall rules in the FORWARD chain, as many administrators assume enabling IP forwarding is sufficient.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure firewall rules to allow forwarding between the interfaces.
To enable a Linux system to route traffic between two directly connected subnets, IP forwarding must be enabled in the kernel, and firewall rules must permit forwarding between the interfaces. Static routes are not needed because the networks are directly connected. NAT is not required for internal routing, and configuring client gateways is done on the clients, not the router.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Add static routes for each subnet on both interfaces.
Why it's wrong here
Since the router is directly connected to both subnets, it already has routes for them in its routing table. Adding static routes for directly connected networks is unnecessary and could cause conflicts. Static routes are needed for remote networks not directly attached. Therefore, this step is not required for routing between the two local subnets.
- ✗
Set the default gateway on both subnets to point to the router's interface IP.
Why it's wrong here
Configuring the default gateway on hosts in each subnet is necessary for them to send traffic to the router, but it is not a step performed on the router itself. The question asks for steps to configure the system as a router, not the clients. The router's configuration involves enabling forwarding and allowing traffic through the firewall. Setting gateways on clients is a separate task.
- ✓
Configure firewall rules to allow forwarding between the interfaces.
Why this is correct
Even with IP forwarding enabled, firewall rules may block forwarded traffic. By default, many distributions have a default deny policy in the FORWARD chain. The administrator must add rules to allow traffic from eth0 to eth1 and vice versa, such as iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT and the reverse. This ensures that packets are not dropped by the firewall.
- ✓
Enable IP forwarding by setting net.ipv4.ip_forward=1.
Why this is correct
Enabling IP forwarding is essential for the kernel to forward packets between interfaces. Without it, the system will not route traffic between the subnets. This can be done at runtime with sysctl -w net.ipv4.ip_forward=1 or persistently by adding the setting to /etc/sysctl.conf or a file in /etc/sysctl.d/. This step is fundamental for any routing configuration.
- ✗
Enable NAT (masquerading) on the external interface.
Why it's wrong here
NAT is used when private IP addresses need to be translated to a public IP for internet access. In this scenario, the router is connecting two private subnets, and NAT is not required for routing between them. NAT would actually hide the source addresses and could break communication if not configured properly. It is not a necessary step for basic routing between two internal subnets.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official LPI exam blueprint
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