Mobile Networking · How Apps Talk · Lesson 1 of 4

Packets & IP Addresses

See how a chat message becomes packets and what an IP address tells you.

Your Message, in Pieces

When you send a chat message, your phone breaks the data into small chunks called packets. Each packet travels across the internet on its own, and the receiving server puts them back together in order.

Every packet carries a header, a bit like the outside of an envelope. The header says where the packet came from and where it's going. Most chat and web traffic uses a protocol called TCP, which also gives each packet a sequence number. The server uses those numbers to rebuild the message and to ask again for any piece that went missing.

The contents of the packet are called the payload. Most apps today encrypt the payload with HTTPS, so anyone who catches the packet on the way sees scrambled bytes. The header still shows the addresses, though. That's why network logs can reveal who talked to whom even when they can't show what was said.

On the simulated phone, send "See you at 7" in the chat app. Tap each of the 3 packets and read its header. Note the destination port.

IP Addresses, Public and Private

An IP address is the number that tells the network where to deliver a packet. The common kind, IPv4, is written as four numbers from 0 to 255 separated by dots, like 203.0.113.10. IPv4 allows about 4.3 billion addresses. That sounds like plenty, but IANA, the body that hands out address blocks, gave away its last free IPv4 blocks in February 2011.

To stretch the supply, some ranges are set aside as private. Home and office networks use them freely, and they're never routed on the public internet. The private ranges are 10.x.x.x, 172.16.x.x to 172.31.x.x, and 192.168.x.x. Your phone at home probably has one, like 192.168.1.23 in the lab. Huge numbers of homes use that same address at once without clashing.

Public addresses are unique across the internet. A server like the chat app's needs one so anyone can reach it.

For investigators, the difference matters. A private address only identifies a device inside one local network. On its own, it says nothing about which home or city the device is in.