Whois cannot find information on a couple IPs, is that okay?

[RagingHippieHost]

There are a couple guests that keep showing up and trying to sign in even though SMF says that there aren't members associated with them.

I was curious and tried to look them up with WHOIS and no results were found. What does this mean? Should I be concerned?

[MH-MINI]

Arin doesn't own all IP addresses.