Overview

• What you’ll learn: How routers and switches operate, the difference between Layer 2 switching and Layer 3 routing, how routing tables work, configuring static routes on Linux, and enabling IP forwarding to use Linux as a router.
• Prerequisites: Lesson 13 – Socket Programming Fundamentals
• Estimated reading time: 20 minutes

Introduction

When a packet leaves your computer, how does it know where to go? The answer lies in routing — the process of selecting paths in a network along which to send data. Routers are the devices (or software) that make these forwarding decisions, using routing tables to determine the best next hop for each packet.

At a lower level, switches operate within a single network segment, forwarding frames based on MAC addresses. Understanding both routing and switching is essential for designing, configuring, and troubleshooting networks.

In this lesson, you will learn the fundamentals of Layer 2 switching and Layer 3 routing, explore how Linux maintains and uses routing tables, configure static routes, and set up a Linux machine as a simple router.

Layer 2 Switching vs Layer 3 Routing

Network traffic is forwarded at two primary layers of the OSI model. Switches operate at Layer 2 (Data Link) and forward Ethernet frames based on MAC addresses. They maintain a MAC address table (also called a CAM table) that maps MAC addresses to physical ports. Routers operate at Layer 3 (Network) and forward IP packets based on destination IP addresses using routing tables.

# View the ARP table (Layer 2 — MAC-to-IP mappings)
$ ip neigh show
192.168.1.1 dev ens33 lladdr 00:50:56:c0:00:08 REACHABLE
192.168.1.5 dev ens33 lladdr 00:0c:29:12:34:56 STALE
192.168.1.254 dev ens33 lladdr 00:50:56:f1:ab:cd REACHABLE

# View the routing table (Layer 3 — IP forwarding decisions)
$ ip route show
default via 192.168.1.1 dev ens33 proto dhcp metric 100
192.168.1.0/24 dev ens33 proto kernel scope link src 192.168.1.10 metric 100
10.0.0.0/8 via 192.168.1.254 dev ens33 metric 200

# View bridge (virtual switch) information
$ bridge fdb show
00:0c:29:ab:cd:ef dev ens33 master br0
ff:ff:ff:ff:ff:ff dev ens33 master br0

How Routing Works

When a host needs to send a packet, it first checks whether the destination is on the local network (same subnet). If it is, the packet is delivered directly via Layer 2. If the destination is on a different network, the packet is sent to the default gateway — typically a router — which then forwards it toward the destination.

Routers make forwarding decisions by consulting their routing table. They match the destination IP address against the table entries using longest prefix match — the most specific route wins. If no specific route matches, the default route (0.0.0.0/0) is used.

# Trace the route a packet takes to reach a destination
$ traceroute -n 8.8.8.8
traceroute to 8.8.8.8 (8.8.8.8), 30 hops max, 60 byte packets
 1  192.168.1.1      1.234 ms   1.123 ms   1.089 ms
 2  10.0.0.1         5.678 ms   5.432 ms   5.321 ms
 3  172.16.0.1      12.345 ms  12.234 ms  12.123 ms
 4  8.8.8.8         15.678 ms  15.432 ms  15.321 ms

# View the routing table in detail
$ ip route show table all | head -20
default via 192.168.1.1 dev ens33 proto dhcp metric 100
192.168.1.0/24 dev ens33 proto kernel scope link src 192.168.1.10

# Check which route a specific destination would use
$ ip route get 10.0.5.100
10.0.5.100 via 192.168.1.254 dev ens33 src 192.168.1.10 uid 1000
    cache

Configuring Static Routes

Static routes are manually configured entries in the routing table. They are useful for small networks, specific traffic paths, or when you need to override dynamic routing decisions.

# Add a static route: traffic for 10.10.0.0/16 goes via 192.168.1.254
$ sudo ip route add 10.10.0.0/16 via 192.168.1.254 dev ens33

# Add a route to a specific host
$ sudo ip route add 203.0.113.50/32 via 192.168.1.1 dev ens33

# Delete a route
$ sudo ip route del 10.10.0.0/16 via 192.168.1.254

# Replace an existing route
$ sudo ip route replace 10.10.0.0/16 via 192.168.1.253 dev ens33

# Verify the routing table
$ ip route show
default via 192.168.1.1 dev ens33 proto dhcp metric 100
10.10.0.0/16 via 192.168.1.254 dev ens33
192.168.1.0/24 dev ens33 proto kernel scope link src 192.168.1.10

# Make static routes persistent with Netplan (Ubuntu)
# Edit /etc/netplan/01-netcfg.yaml:
#   ethernets:
#     ens33:
#       routes:
#         - to: 10.10.0.0/16
#           via: 192.168.1.254

Linux as a Router

Any Linux machine with two or more network interfaces can act as a router by enabling IP forwarding. This tells the kernel to forward packets between interfaces instead of dropping them.

# Check if IP forwarding is enabled (0 = disabled, 1 = enabled)
$ sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 0

# Enable IP forwarding temporarily
$ sudo sysctl -w net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1

# Enable IP forwarding permanently
$ echo "net.ipv4.ip_forward=1" | sudo tee -a /etc/sysctl.d/99-router.conf
$ sudo sysctl --system

# Example: Linux router with two interfaces
# ens33: 192.168.1.10/24 (internal network)
# ens34: 203.0.113.10/24 (external network)
#
# Enable forwarding and NAT:
$ sudo sysctl -w net.ipv4.ip_forward=1
$ sudo iptables -t nat -A POSTROUTING -o ens34 -j MASQUERADE
$ sudo iptables -A FORWARD -i ens33 -o ens34 -j ACCEPT
$ sudo iptables -A FORWARD -i ens34 -o ens33 -m state --state RELATED,ESTABLISHED -j ACCEPT

# Verify forwarding is working
$ cat /proc/sys/net/ipv4/ip_forward
1

Key Takeaways

• Switches forward frames at Layer 2 using MAC addresses; routers forward packets at Layer 3 using IP addresses and routing tables.
• The routing table uses longest prefix match to select the best route; if no specific route matches, the default gateway is used.
• Static routes are configured with ip route add and can be made persistent through Netplan configuration on Ubuntu.
• Linux can function as a router by enabling IP forwarding (net.ipv4.ip_forward=1) and configuring NAT with iptables.
• Tools like traceroute, ip route get, and ip neigh show help diagnose routing and switching issues.

What’s Next

In Lesson 15, you will learn Network Configuration with Netplan — Ubuntu’s declarative YAML-based network configuration system for managing interfaces, addresses, routes, and DNS settings.