IP Routing and Subnetting with CIDR on Linux

IntermediateNetworking~4 hours
CIDRipcalcroutingforwarding

I subdivided a /24 into role-based CIDR subnets and configured a Linux host as a router to forward traffic between them. Correct subnet sizing and route tables produced verified end-to-end reachability with no overlap or black-hole routes.

Objective & Context

Subnetting controls broadcast domains and enforces segmentation; routing connects those segments. This lab plans address space with VLSM and turns a multi-homed Linux box into a forwarding router, the foundation for the VLAN and zero-trust segmentation labs.

Environment & Prerequisites

  • A Linux router VM with two interfaces; two client VMs.
  • ipcalc for subnet math; iproute2 for routes.
  • A /24 to subdivide (for example 192.168.10.0/24).

Step-by-Step Execution

1. Plan subnets with ipcalc

ipcalc 192.168.10.0/24 --split 60 60

2. Enable IP forwarding [PRIVILEGED]

sysctl -w net.ipv4.ip_forward=1 && echo 'net.ipv4.ip_forward=1' >> /etc/sysctl.conf

3. Add and inspect routes

ip route add 192.168.10.64/26 via 192.168.10.62 && ip route show
192.168.10.0/26 dev eth0 proto kernel scope link
192.168.10.64/26 via 192.168.10.62 dev eth1

Validation & Testing

From a client in subnet A, ping a client in subnet B and trace the path through the router. Pass criteria: successful cross-subnet ping, the router appears as the first hop, and no overlapping routes exist.

traceroute 192.168.10.70
Advanced: Troubleshooting
  • No cross-subnet traffic: confirm ip_forward=1 and that clients use the router as gateway.
  • Overlapping subnets: recompute with ipcalc; overlaps cause unpredictable routing.
  • Asymmetric routing: ensure return routes exist on both segments.

Key Results

  • Segmented one /24 into 2 role-based /26 subnets with zero overlap.
  • Achieved verified bidirectional reachability across segments.
  • Reduced each broadcast domain by roughly 75% versus a flat /24.
  • Documented a reusable VLSM plan for future segments.

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