Earlier quoted context omitted.
I see 1.2.3.4 all the time. It is clearly Sprint (in the U.S., mobile connection) doing it. The urls have the same 'bmi' stuff mentioned above, apparently they are using a similar implementation.
I did a bit of searching and it seems a few different mobile carriers are doing this with the 1.2.3.0/24 range, but the important thing is they should not be doing it at all. It would break the Internet if everyone just used whatever address they felt like using.
I was mostly trying to confirm that it was the carriers and not malware or whatever.