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Every Linux system that communicates over a network needs at least one network interface configured with an IP address, a subnet mask, and a default gateway. Linux gives you several layers of tooling to manage this: the modern ip command for live inspection and temporary changes, Netplan for persistent declarative configuration on Ubuntu, and NetworkManager for desktop or cloud environments. Understanding how these tools fit together helps you diagnose connectivity issues and apply changes that survive a reboot.

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Viewing Network Interfaces

Before making any changes, inspect your current interface state to understand what the system already has configured. 
ip addr show                       # list all interfaces
ip addr show eth0                  # specific interface
ip link show                       # link layer info
ifconfig                           # (legacy, may need net-tools)
Interface names vary by system. Older systems use eth0, while modern Linux kernels use predictable names such as ens3, enp3s0, or eno1. The name depends on your hardware slot, firmware, and udev rules. Run ip link show to see the exact names on your machine before referencing them in any configuration file.

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Viewing Routes

Use the following commands to inspect the routing table and understand how traffic leaves your system. 
ip route show
ip route get 8.8.8.8               # route for specific host
ip route show table all            # show all routing tables

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Temporary IP Changes

The ip command applies changes immediately but they do not persist across reboots. Use these commands for testing or quick fixes. 
sudo ip addr add 192.168.1.10/24 dev eth0
sudo ip addr del 192.168.1.10/24 dev eth0
sudo ip link set eth0 up
sudo ip link set eth0 down
sudo ip route add default via 192.168.1.1
Temporary changes made with ip are lost when the system reboots or the network service restarts. Always follow up with a persistent configuration (Netplan or NetworkManager) if you need the settings to survive.

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Persistent Configuration with Netplan

Ubuntu 17.10 and later use Netplan for persistent network configuration. Edit or create a file under /etc/netplan/ to define your desired state. 
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      dhcp4: false
      addresses:
        - 192.168.1.100/24
      routes:
        - to: default
          via: 192.168.1.1
      nameservers:
        addresses:
          - 8.8.8.8
          - 8.8.4.4
After saving the file, apply the configuration: 
sudo netplan apply
Run sudo netplan try before sudo netplan apply to test the configuration with an automatic rollback after 120 seconds. This prevents you from locking yourself out of a remote machine if the new settings are incorrect.

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Managing Connections with NetworkManager

Desktop systems and many cloud images use NetworkManager. The nmcli command-line tool lets you inspect and modify connections without a GUI. 
nmcli device status
nmcli connection show
nmcli connection up "My Connection"
nmcli con mod "My Connection" ipv4.addresses 192.168.1.100/24
nmcli con mod "My Connection" ipv4.gateway 192.168.1.1
nmcli con mod "My Connection" ipv4.method manual
After modifying a connection, bring it back up to apply the changes: 
nmcli connection up "My Connection"

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Useful Diagnostics

Use these commands to verify connectivity, trace the path to a destination, and inspect open sockets. 
ping -c 4 8.8.8.8
traceroute 8.8.8.8
ss -tuln                           # listening sockets
netstat -tuln                      # (legacy)
Prefer ss over netstat on modern systems — it is faster, reads directly from kernel socket structures, and is actively maintained. netstat requires the net-tools package and is considered legacy.