Servers sometimes need more than one IP address: separate IPs for different SSL sites on old clients, a dedicated IP for a mail server’s reputation, services that must listen on their own address, or outbound traffic from a specific IP. Once your provider has assigned the additional IPs to your server, you configure them on the network interface. Here’s how on each common setup.

First, get the addresses from your provider. Adding an IP that isn’t routed to your server doesn’t work, and can disrupt other customers. On VPS.DO, additional IPv4 addresses for multi-IP VPS use are assigned to your service, and you configure them inside the OS. Note the netmask and gateway details your provider gives you.

Check the current configuration

ip -br addr
ip route

Find the interface name (eth0, ens3, enp1s0…) and the existing primary address. See ip command.

Temporary: ip addr (lost at reboot)

Useful for testing:

sudo ip addr add 203.0.113.21/32 dev eth0
ip addr show eth0
ping -c 3 -I 203.0.113.21 1.1.1.1      # test outbound from the new IP

Remove it again:

sudo ip addr del 203.0.113.21/32 dev eth0

Use the prefix length your provider specifies: /32 for a single routed IP, or the subnet’s prefix (e.g. /24 or /29) if the IPs are part of an on-link subnet.

Ubuntu: Netplan

Edit the existing file in /etc/netplan/ (e.g. 50-cloud-init.yaml or 01-netcfg.yaml):

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 203.0.113.10/24        # primary
        - 203.0.113.21/32        # secondary
        - 203.0.113.22/32
      routes:
        - to: default
          via: 203.0.113.1
      nameservers:
        addresses: [1.1.1.1, 9.9.9.9]

Apply it safely: netplan try rolls back automatically if you lose connectivity and don’t confirm:

sudo netplan try
sudo netplan apply

If the file is generated by cloud-init, stop cloud-init from overwriting your changes. Create /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with network: {config: disabled}. See netplan and cloud-init.

Debian: ifupdown (/etc/network/interfaces)

auto eth0
iface eth0 inet static
    address 203.0.113.10/24
    gateway 203.0.113.1

# secondary addresses
iface eth0 inet static
    address 203.0.113.21/32

iface eth0 inet static
    address 203.0.113.22/32

Apply it carefully. Restarting networking over SSH can cut your connection:

sudo ifup eth0        # or reboot during a maintenance window

Keep the VNC console open in case something goes wrong. See Linux rescue mode.

RHEL family: NetworkManager (nmcli)

nmcli connection show
sudo nmcli connection modify "System eth0" +ipv4.addresses 203.0.113.21/32
sudo nmcli connection modify "System eth0" +ipv4.addresses 203.0.113.22/32
sudo nmcli connection up "System eth0"

Use the extra IPs

Nginx: bind a site to a specific IP:

server {
    listen 203.0.113.21:443 ssl;
    server_name shop.example.com;
    …
}

With SNI, every modern client can serve many HTTPS sites from one IP. Dedicated IPs mainly matter for specific requirements (legacy clients, IP-based allow-lists, separation). See Nginx server blocks.

Mail server: send from a dedicated IP with its own PTR record:

# /etc/postfix/main.cf
smtp_bind_address = 203.0.113.22

Set reverse DNS for that IP to match the mail hostname. See PTR records.

Outbound traffic from a specific IP:

curl --interface 203.0.113.21 https://ifconfig.me

Applications often have a “bind address” or “source address” option. For system-wide changes, set src on the default route (ip route change default via 203.0.113.1 src 203.0.113.21).

Docker: publish a container port on one IP only: -p 203.0.113.21:443:443. See Docker networking.

Firewall

UFW rules apply to all addresses unless you specify one:

sudo ufw allow to 203.0.113.21 port 443 proto tcp

Verify from outside

ping 203.0.113.21
nc -zv 203.0.113.21 443

If the IP answers locally but not from outside, check with your provider that it’s routed to your server, and that the prefix length and gateway are correct.

Quick reference

Task Command / file
Show addresses ip -br addr
Add temporarily ip addr add IP/32 dev eth0
Ubuntu (persistent) /etc/netplan/*.yaml + netplan try
Debian (persistent) /etc/network/interfaces
RHEL (persistent) nmcli connection modify … +ipv4.addresses
Outbound from IP curl --interface IP …

Related: multi-IP VPS and IPv4 vs IPv6.

  • Networking
  • Linux
  • IP Addresses