IPv4 Blocks for Private Internets
The IPv4 blocks
10.0.0.0 - 10.255.255.255 (10/8 prefix) 172.16.0.0 - 172.31.255.255 (172.16/12 prefix) 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
are reserved as private address space, specified in the RFC 1918. This means everybody can set up an internal network using addresses in these blocks, but packets originating from those addresses should never appear on the internet.
Consequently, the administrators of those internal networks are not obliged to register themselves with a whois database such as the RIPE Database.
Thus, if you have received packets originating from these addresses, there is either a misconfiguration somewhere or the packets that you received contain a spoofed IP address, or they have been sent from within your own network.
RFC 1918 can be found at ftp://ftp.ripe.net/rfc/rfc1918.txt