First: Bumping very old topic I know, but I believe still very relevant to clarify. Also, this forum topic was top on my searchlist when searching on this subject ;-)
The reason I came upon this thing, was my firewall(pfsense) log, getting filled up with these entries every 3 seconds.
I have also struggled to find out what the heck this was. I have spent loads of time on it. In the end, trial and error, leaving out every network part 1 by 1.
I found out, it comes from the ISP local-internet(whats it called?), all the isp-costumers, and when someone's modem is starting up, it sends dhcp request out on the network, speeding right through my isp-modem because it is in bridge mode, and ends up on the WAN-side of my pfsense router. Therefore the block bogon(private range ip-adresses), in the firewall log.
In pfsense firewall and settings, I can tick off to NOT log those "block bogon" packets, if I those to.
RafterX out.