This error occurs due to temporary failure in resolving the domain 'ports.ubuntu.com'

Question

root@ubuntu:/# sudo apt update
Err:1 http://archive.ubuntu.com/ubuntu focal InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:2 http://security.ubuntu.com/ubuntu focal-security InRelease       
  Temporary failure resolving 'security.ubuntu.com'
Err:3 http://ports.ubuntu.com/ubuntu-ports focal InRelease             
  Temporary failure resolving 'ports.ubuntu.com'
Err:4 http://archive.ubuntu.com/ubuntu focal-updates InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:5 http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease
  Temporary failure resolving 'ports.ubuntu.com'
Err:6 http://archive.ubuntu.com/ubuntu focal-backports InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:7 http://ports.ubuntu.com/ubuntu-ports focal-backports InRelease
  Temporary failure resolving 'ports.ubuntu.com'
Err:8 http://ports.ubuntu.com/ubuntu-ports focal-security InRelease
  Temporary failure resolving 'ports.ubuntu.com'
Reading package lists... Done
Building dependency tree       
Reading state information... Done
All packages are up to date.
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal-updates/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal-backports/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/focal-security/InRelease  Temporary failure resolving 'security.ubuntu.com'
W: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal/InRelease  Temporary failure resolving 'ports.ubuntu.com'
W: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal-updates/InRelease  Temporary failure resolving 'ports.ubuntu.com'
W: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal-backports/InRelease  Temporary failure resolving 'ports.ubuntu.com'
W: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal-security/InRelease  Temporary failure resolving 'ports.ubuntu.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.