Managing IP addresses in Debian/Ubuntu

This applies to Ubuntu 18.04 and 20.04, Debian 9 and 10.
Other versions may work, but I have not tested

Access the server via SSH

Edit the file /etc/network/interfaces

nano /etc/network/interfaces

Replace the contents with this:

# The loopback network interface
auto lo ens192
iface lo inet loopback

# The primary network interface
#Replace the letters with your IP. NOTE THE ZERO ON THE NETWORK LINE!!!
auto ens192
allow-hotplug ens192
iface ens192 inet static
     address ABC.DEF.GHI.JKL
     network ABC.DEF.GHI.0
     netmask 255.255.255.255
     broadcast ABC.DEF.GHI.JKL
     gateway 10.255.255.1

#Enable the next few lines to add extra IPs. The UP and DOWN go in pairs
#     up   ip addr add 77.68.127.176/32 dev ens192 label ens192:0
#     down ip addr del 77.68.127.176/32 dev ens192 label ens192:0

#     up   ip addr add 109.228.36.59/32 dev ens192 label ens192:1
#     down ip addr del 109.228.36.59/32 dev ens192 label ens192:1

#enable the following section for ipv6
#iface ens192 inet6 static
#    address 2a00:da00:1800:81e8::1/128
#    netmask 128
#    gateway fe80::1
#    accept_ra 0
#
#Enable the next few lines to add extra IPs. The UP and DOWN go in pairs
#    up   ip -6 addr add 2a00:da00:1800:81e8::2/128 dev ens192 label ens192:2
#    down ip -6 addr del 2a00:da00:1800:81e8::2/128 dev ens192 label ens192:2

#    up   ip -6 addr add 2a00:da00:1800:81e8::3/128 dev ens192 label ens192:3
#    down ip -6 addr del 2a00:da00:1800:81e8::3/128 dev ens192 label ens192:3

Save (Ctrl-X then Y)

Check your /etc/resolv.conf file to establish where to place your DNS

A file that contains one or more IPs, is a normal resolv.conf file and you can leave it alone or add ipv6 DNS as you wish

nameserver 8.8.8.8
nameserver 2001:4860:4860::8888

A file that contains 127.0.0.X indicates your DNS is managed by systemd. Leave this alone and edit as follows:

nano /etc/systemd/resolved.conf

#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.
#
# Entries in this file show the compile time defaults.
# You can change settings by editing this file.
# Defaults can be restored by simply deleting this file.
#
# See resolved.conf(5) for details

[Resolve]
DNS=2001:4860:4860::8844 2001:4860:4860::8888 8.8.8.8 8.8.4.4
#FallbackDNS=
#Domains=
#LLMNR=no
#MulticastDNS=no
#DNSSEC=no
#Cache=yes
#DNSStubListener=yes

Don't add the Ipv6 addresses unless you have an ipv6 address to use.

Run:

systemctrl restart networking

Check with:

ip a #List assigned IPs

ping google.com #ensure you can reach google

ping6 google.com #ensure you can reach google over ipv6