Error :unable to resolve host

This may be because if we change the hostname of the Linux system  after installation .
Resolution:

#vim /etc/hosts   and make it as same as below

127.0.0.1 localhost

127.0.1.1 icarus-laptop

# The following lines are desirable for IPv6 capable hosts

::1 ip6-localhost ip6-loopback

fe00::0 ip6-localnet

ff00::0 ip6-mcastprefix

ff02::1 ip6-allnodes

ff02::2 ip6-allrouters

ff02::3 ip6-allhosts

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