Add Multiple IP Addresses into One Single Network Interface. (Server)

The first reason is just because you can ..., and yes Ubuntu server allows you to add multiple virtual IP addresses on a single network interface card without having to buy an additional network adapter.

Let's take the scenario where you are testing a Linux system and need two or more network cards.
Would you purchase a new one?
...
..
.
No, ...not necessary.

You may configure a network card with various IP series, such as 192.168.1.4, 192.168.2.2, 192.168.3.12, etc., and utilize each of them simultaneously.

Sounds helpful? – Of course, it is!

Permanently add an additional IP Address

Ubuntu uses the Netplan utility to configure networking.
We will permanently add an additional IP address to our system by editing the file /etc/netplan/50-cloud-init.yaml.

Let's find the IP address we are currently using:

$ ip addr  

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether xx:xx:32:xx:4x:x4 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.93/24 brd 192.168.1.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 xxzz::xxx:3xxf:fxx5:2xx7/64 scope link 
       valid_lft forever preferred_lft forever
3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether xx:xx:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff
4: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 500
    link/none 
    inet 10.8.0.1/24 scope global tun0
       valid_lft forever preferred_lft forever
    inet6 fxxx::xxx:cxxx:xxxx:axxx/64 scope link stable-privacy 
       valid_lft forever preferred_lft forever

As you see above, the IP address of my network card eth0 is 192.168.1.19.
Now; what if we need to add another IP address, for example, 192.168.1.4?

Let's take a look at our current Netplan configuration and apply the change.

$ sudo cat 50-cloud-init.yaml                                         
[sudo] password for snubmonkey: 

# This is the network config written by 'subiquity'
network:
  ethernets:
     eth0:
     dhcp4: no
     dhcp6: no
      addresses:
      - 192.168.1.93/24
      - 192.168.1.4/24 <<<
      routes:
       - to: default
         via: 192.168.1.1
      nameservers:
        addresses:
        - 9.9.9.9
        search:
        - 208.67.222.222, 1.0.0.1
  version: 2

Here’s a brief explanation of each parameter:

• eth0: A device name to be configured.
• dhcp4: Used to enable or disable dhcp4.
• dhcp6: Used to enable or disable dhcp6.
• addresses: The IP address of the device.
• gateway4: The IP address of your gateway.
• nameservers: The IP address of your DNS server.

Save and close the file when done.
Enable the new configuration with the following command.

$ netplan apply

Restart the networking service.

$ sudo systemctl status systemd-networkd.service

Run the below command to check whether the new IP has been taken into account:

$ ip addr

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether xx:xx:xx:xx:xx:ax brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.93/24 brd 192.168.1.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet 192.168.1.4/24 brd 192.168.1.255 scope global secondary eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::xxxx:xxxx:fxx4:4xxx/64 scope link 
       valid_lft forever preferred_lft forever
3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether xc:x6:3x:x4:4x:xx brd ff:ff:ff:ff:ff:ff
4: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 500
    link/none 
    inet 10.8.0.1/24 scope global tun0
       valid_lft forever preferred_lft forever
    inet6 fe80::xbxx:cxxx:x4xx:ax7x/64 scope link stable-privacy 
       valid_lft forever preferred_lft forever2

Let's double-check by pinging our new assignment address.

-a : the system plays a sound when there is a response from a host. An audible ping is useful when you are troubleshooting network issues and do not want to look at the screen until there is a response.

-c : automatically stops after it sends a certain number of packets, use -c , and a number. This sets the desired amount of ping requests.

ps: as we may have noticed we used the ip addr in favor of the ifconfig command to view the network interface and IP address.

ip addr and ip a do the same.

BONUS
Pros and cons of ip and ipconfig

Since it employs Netlink sockets rather than ioctl system calls, the ip command is more flexible and technically effective than the ifconfig.
We will talk about this more in another of our posts; so stay tuned.

Here is an example:

$ ifconfig

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.93  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 xxxx::xxxx:xxxx:xx1x:xxax  prefixlen 64  scopeid 0x20<link>
        ether dc:a6:32:14:4e:a7  txqueuelen 1000  (Ethernet)
        RX packets 9713030  bytes 6030523854 (6.0 GB)
        RX errors 0  dropped 89  overruns 0  frame 0
        TX packets 8998304  bytes 6420042303 (6.4 GB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

...
..
.

Have you spotted it?

Using ifconfig command does not allow us to see whether or not our NIC is bound to multiple IP addresses.