You can use grep:
grep -Eo '[^.]+\.[^.]+$' file.txt
Example:
$ cat file.txt
sub1.domain.com
sub2.domains2.com
domain3.com
sub5.domain.ext
subof.subdomain.domainx.ex2
$ grep -Eo '[^.]+\.[^.]+$' file.txt
domain.com
domains2.com
domain3.com
domain.ext
domainx.ex2
Note that this will return co.uk for www.google.co.uk.