How to add additional static IPs

Find the interface holding your current IP

ip a

Current distributions assign predictable interface names such as enp1s0 or ens3 rather than the old eth0 style. You may still see eth0 on older images or inside some containers; use whatever name ip a shows for your server.

Find the connection name

nmcli connection show

Add the secondary address

Replace the connection name and address with your own values:

sudo nmcli connection modify "System enp1s0" +ipv4.addresses 192.168.1.51/24
sudo nmcli connection up "System enp1s0"

Edit the Netplan configuration

Open the YAML file in /etc/netplan/ (the filename varies by image, for example 01-netcfg.yaml or 50-cloud-init.yaml):

sudo nano /etc/netplan/01-netcfg.yaml

Add the secondary address to the existing addresses list for your interface:

network:
  version: 2
  renderer: networkd
  ethernets:
    enp1s0:
      dhcp4: no
      dhcp6: no
      addresses: [192.168.1.50/24, 192.168.1.51/24]
      routes:
        - to: default
          via: 192.168.1.1
      nameservers:
        addresses: [9.9.9.9, 1.1.1.1]

Recent Netplan releases no longer use the gateway4 key, so the default route goes in a routes: block as shown above. The example uses two well-known public resolvers so the config is complete and testable. Use a public resolver you trust, or leave DNS on the default your server already provides. See Worldstream DNS servers.

Netplan's YAML is sensitive to indentation and spacing. Keep the existing structure and only add to the addresses list.

Apply the configuration

sudo netplan try

netplan try applies the configuration, then asks you to confirm it. If you do not confirm within the timeout, it rolls back automatically, so a broken file cannot leave you locked out of your own server. Use sudo netplan apply only when you are certain the file is correct.