IPs & Interfaces

1. Finding Your Network Interfaces

Before configuring anything, you need to know the name of your network card (e.g., enp0s3).

• Quick Check: ls /sys/class/net.
• The “Standard” Way: ip addr (or ip a).
• Detailed Hardware Info: sudo lshw -class network.
• Legacy/Old Way: ifconfig.

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2. The ip Command (The Modern Standard)

The ip utility is used for immediate, temporary changes that disappear after a reboot.

Command Format

ip [options] object command [parameters]

Top Objects & Commands

• Objects: address (or a), link (or l), route (or r).
• Common Options:
  • br (Brief): Summary for readability.
  • 4 / 6: Show only IPv4 or IPv6.
  • s (Stats): Show traffic statistics.

Key Usage Examples

• Add IP: sudo ip addr add 192.168.1.200/24 dev enp0s3.
• Bring Interface Up/Down: ip link set dev enp0s3 up (or down).
• Set Gateway: sudo ip route add default via 10.1.1.1.
• Clear All IPs: ip addr flush enp0s3.

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3. Permanent Configuration

To make changes survive a reboot, you must edit configuration files.

Ubuntu (Netplan)

Uses YAML files in /etc/netplan/*.yaml.

Warning: No tabs allowed! Use only spaces.

• To Apply: sudo netplan apply.
• To Test: netplan try (reverts if you lose connection).

NOTE: An Interface can have multiple address

CentOS (Network Scripts)

Uses text files in /etc/sysconfig/network-scripts/ifcfg-[interface].

• To Apply: systemctl restart network (Note: Modern CentOS/RHEL 9 uses nmcli instead).

• Key File Settings:

  • BOOTPROTO="none" (for Static) or "dhcp".
  • ONBOOT="yes" (starts on boot).
  • NOTE: network and above script config is Depricated in new versions of RedHat.. its moved to /etc/NetworkManager/system-connections

  nmcli (NetworkManager CLI)

  Best for servers and scripts. It controls the NetworkManager service directly.

  • Show status: nmcli device or nmcli connection show.
  • Modify connection: nmcli con mod [name] +ipv4.addr "10.0.5.20/24".
  • Activate changes: nmcli con up [name].

  

  

  

  

  N/w Modification..

  • nmcli con mod <CON_NAME> +/- ipv4.addr “IP ADDR”
  • nmcli con up/down <CON_NAME>

  

  OLD stuff can be managed by ifcfg from sudo dnf install network-scripts -y

  

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DNS & Hostnames

• Hostname: Use hostnamectl set-hostname [name].
• DNS (Manual/Temporary): Edit /etc/resolv.conf and add nameserver 8.8.8.8.
• DNS (Ubuntu/Modern): Configured via the nameservers section in your Netplan YAML.
• Troubleshooting: systemd-resolve --status shows current active DNS servers.
• For flushing DNS cash may be used command: systemd-resolve --flush-caches
• For setting DNS server address may be used command: systemd-resolve --set-dns=SERVER

• /etc/resolv.conf: This is the legacy standard. Almost every Linux application is hard-coded to look at this specific file to find its DNS instructions.
• /run/systemd/resolve/resolv.conf: This is the modern dynamic file. It is generated automatically by the systemd-resolved service and contains the “real” DNS servers your network is currently using.

Feature	ip Command	nmcli (NetworkManager)
Persistence	Temporary. Changes take effect immediately but are lost after a reboot.+1	Permanent. Changes are saved to configuration files and survive a reboot.+1
Object Type	Works with low-level system objects like address, link, and route.+2	Works with high-level connections and devices.+1
Usage	Ideal for quick testing or one-time troubleshooting.	Ideal for servers and persistent network setups.+1
Syntax	ip [options] object command [parameters].	`nmcli [options] OBJECT { COMMAND

• lshw → for in-depth details

  

  

• to change any of the above details… we can use ethtool

  

  

• sudo hostname <NEW NAME> OR hostname