What could be reason behind "wget: unable to resolve host address ‘archive.ubuntu.com’" error

I'm trying to download the netboot ubuntu 18.04 image and using this link

It has directories and file directly. So trying to download all the files and sub-directory recursively... For that, I'm using following command:

wget -m http://archive.ubuntu.com/ubuntu/dists/bionic-updates/main/installer-amd64/current/images/netboot/ubuntu-installer/

which gives following error:

--2020-03-16 14:23:12--  http://archive.ubuntu.com/ubuntu/dists/bionic-updates/main/installer- 
amd64/current/images/netboot/ubuntu-installer/
Resolving archive.ubuntu.com (archive.ubuntu.com)... failed: Temporary failure in name resolution.
wget: unable to resolve host address ‘archive.ubuntu.com’

Not sure what is the reason behind this error, any help/direction is appreciated!