IP to Domain name not resolved

I am tyring the 'host IPADDRESS' command at the prompt and I am getting a 'not found' error. This means that the domain name of the IP Add. cannot be resolved. In practical terms, what does mean and why is it imp?

i.e. the ip address has no dns name assigned to it.

the DNS system is somewhat one-way. any DNS name has to have and IP assigned to it. but not every IP has to have a DNS name...

this is common use for ISPs that supply their customers with dynamic IPs, but imho it´s bad. my ISP uses a DNS name like "ppp-providername-ipaddress.provider-name.de" which makes my dynamic IP work with several services that depend on the "reverse DNS resolution".

Thanks for the info.
I have created a mail filter and one of the filters is that if the IP address cannot be resolved to a DNS name, then to drop the email. Is this a good way to suppose that this user could be a spammer?

reading my own post again... no
on the other hand - you could test if the reverse-DNS is equal to the DNS resolved name and if the reverse-DNS is nothing, you still allow it.

security is an odd issue... for a personal website, i would disallow it (i like security), but for a commercial website, i would probably allow it (they would like as many people as possible to use their service).

hard to tell, really.