Are udp packets with non-routeable ip addresses valid on public network?

[Ernie Luzar]

Hello List

I am receiving unsolicited inbound udp packets with a "to ip address" 
[10.0.10.1] of a computer on my LAN. Is this valid? Other tcp/udp 
packets from that LAN computer pass through the firewall NAT as 
expected. I added a firewall rule to block that packet and their are no 
outward signs of problems with that LAN computer.

On other LAN PC's that run ms/windows and facebook or yahoo are sending 
out bound udp packets with "from ip address" containing their LAN ip 
address. I bock these also without any outward signs of problems. These 
packets are not being NAT'ed like other udp packets from that LAN PC are.

I though non-routeable ip addresses are invalid on the public network.

Any ideas on what is occurring here?