Mobile Networking · Mobile Data Under the Hood · Lesson 5 of 5

Challenge: Online on the Train

Get a friend's phone online, share it, and keep notifications flowing.

The Case Brief

You're on a long train ride with your friend Sam. Sam just switched to a budget carrier and put its SIM into an unlocked Android phone bought somewhere else.

Sam is frustrated. Calls go through fine, and texts arrive. But the moment Sam opens a browser or a map, nothing loads. The phone shows full signal bars, and mobile data is switched on in Settings.

You check a few things together. The phone isn't in airplane mode. The plan includes plenty of data, and the carrier's website says the account is active. Sam's old SIM from the previous carrier worked perfectly in this same phone last week.

The train's own Wi-Fi is down, so mobile data is your only way online. You want to find the most likely cause before Sam spends an hour on hold with support.

Sharing the Connection

You enter the APN from the carrier's help page, and Sam's data starts working. Sam turns on a Wi-Fi hotspot so your laptop can get online too.

On your laptop, you open a website that shows your public IP address. You're curious which address it will show, since several are involved: the private address the hotspot gave your laptop, the address in Sam's phone settings, and whatever address Sam's connection uses out on the internet.

Only IPv6

Sam's phone settings list only an IPv6 address, something like 2001:db8:4a::1c. There's no IPv4 address on the cellular connection at all.

Sam opens an old game that connects straight to an IPv4 address, with no name lookup. You expect DNS64 and NAT64 alone would fail here. But the game works.

Something on Sam's Android phone must be turning the game's IPv4 packets into IPv6 before they leave, so the carrier can turn them back into IPv4 for the internet.

Waiting for a Message

Later the train's Wi-Fi comes back, and you switch your own iPhone over to save Sam's data. You notice the train network's login page says it blocks some ports, including 5223, but allows normal web traffic.

You're expecting an important chat message from your family. Your iPhone gets notifications for every app through one shared connection to Apple's push service. You wonder if you'll miss the message while you're on this network.