IP aliasing

His case isn't unique. My configuration uses a business grade docsis cable modem with all routing features disabled. It hands a /29 and IIRC a /60 to my firewall. The modem uses 10.0.10.0 for configuration that none of my internal networks use this subnet. The cable modem will respond to requests from off subnet, including on a 192.168 address. I haven't tested this but I think they'll respond to any request on their default IP as long as it originates from the client side. It totally ignores any subnetting issues or configurations. The only way I can prevent this is to drop all packets to from that particular IP on the firewall. I was boggled the first time I typed in the address on the local lan before I'd creating a routing rule and not only did the modem respond the firewall routed it. I've never understood why it was doing that.

However they are doing it, I suspect it's done like this so they field less calls about why someone's cable modem won't respond when they type in the default address even if their network config is completely different.