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.

Well if they are doing a mixed private/public net then it wouldn't be like the addresses are routable outside of their network. I've seen a number of clients which were essentially 'natted' behind the ISPs infrastructure. At its core the ISP gets all packets landing at their router and if they want to advertise an 'inside' route to 1.2.3.4 (or 10.0.0.1) that is something they could do successfully.

Yep, you're right; a net with mixed public/private addresses can certainly work well when done correctly. Unfortunately, it can also seem to work for some period of time when done incorrectly. The trouble is, most people don't grasp the ramifications of doing it wrong. I'm sure you understand the ramifications at least as well as I do, and probably a whole lot better, but for the sake of everyone else in the room...

When a network/ISP misuses unallocated address space by routing the traffic to something internal, this prevents the inside of the network/ISP from reaching those addresses normally. Unallocated address space can be allocated by the RIR's at any time, so misuse of unallocated address space results in parts of the Internet unreachable.

If some huge networks/ISP's (Comcast, Verizon, Sprint, ...) decided to misuse the address block allocated to you for some internal purpose, you'd be rightfully upset since it would prevent all users of those ISP's from reaching your service/servers. Now let's assume you're a new company and just got a new allocation of addresses from the RIR only to find out the users of major ISP's can't reach your service because the ISP's have already misused your address block for something internal on their networks. Yep, you'd be livid, and livid with good reason.

If you put a lot of work into your misuse of unallocated address space, all that effort could turn out to be wasted a few hours later when the block you misused gets allocated. To reach the newly allocated block, you'd need to redo all that work over again, correctly.

Yeah, I'd prefer they weren't doing content aware stuff (if they cared it would probably be optional, so I'm not surprised that they are doing it poorly).

I was mostly trying to confirm that it was the carriers and not malware or whatever.

Thanks for the links!

This is T-Mobile mangling stuff. They do some other things which are annoying, but not Internet breaking. It's a sub-optimal Internet experience, but pretty handy for what I want to do.