To build on Martin’s answer: If you do a DNS lookup on (say) logmein.com you’ll get one ore more IP addresses. When you do a reverse lookup on these addresses they may not map back to the expected domain (i.e. logmein.com in this example).
If you run a Unix box (or have other tools for DNS queries) you can test this using the dig command. In this particular case
yields three IP addresses (from where I am):
logmein.com. 149 IN A 126.96.36.199
logmein.com. 149 IN A 188.8.131.52
logmein.com. 149 IN A 184.108.40.206
And a reverse lookup:
dig -x 220.127.116.11
reveals that logmein.com is running on Microsoft’s azure systems and that there is no FQDN associated with the IP address.
Thus filtering traffic on the domain name logmein.com will not work.