3.1 Sharing the Air


When your laptop or phone is using WiFi to connect to the Internet, it is sending and receiving data with a small, low-powered radio.
The radio in your computer can only send data about 300 meters, so your computer sends your packets to the router in your home, which forwards the packets using a link to the rest of the Internet.
Sometimes we call the first router that handles your computer’s packets the “base station” or “gateway”.
All computers that are close enough to the base station with their radios turned on receive all of the packets the base station transmits, regardless of which computer the packet is supposed to be sent to. They also “hear” all the packets sent by every other nearby computer. So your computer needs a way to know which packets to treat as its own and which packets are being sent to other computers and can be safely ignored.
An interesting side effect of the fact that all the computers within range can hear all packets is that a rogue computer could also be listening to and capturing your packets, perhaps getting ahold of important data like bank account numbers or passwords to online services. We will come back to the issue of protecting your data from prying eyes and ears in a later section.
Every WiFi radio in every device that is ever built is given a unique serial number at the time it is manufactured. This means that each of the computers using WiFi has its own serial number, and the radio in the gateway also has a serial number. You can usually go into a settings screen on your device and look up the serial number for the WiFi radio in your device. It is generally shown in the following form:

This is just a representation of a 48-bit serial number for your WiFi radio. It is also called the “Media Access Control” or “MAC” address. A MAC address is like a “from” or “to” address on a postcard. Every packet (radio postcard) sent across the WiFi has a source and destination address, so all of the computers know which messages are theirs.
When you turn on your computer and connect to a WiFi network, your computer needs to figure out which of the MAC addresses on the WiFi can be used to send packets to the router. When you move from one physical location to another, your computer will be talking to different gateways and each of those gateways will have a different serial number. So when you first connect to a new WiFi, your computer must discover the MAC address for the gateway of that particular WiFi.
To do this, your computer sends a special message to a broadcast address, effectively asking the question, “Who is in charge of this WiFi?” Since your computer knows it is not the gateway itself, it sends a broadcast message with its own serial number as the “from” address and the broadcast address as the “to” address to ask if there are any gateways present on the WiFi network.

From: 0f:2a:b3:1f:b3:1a
To: ff:ff:ff:ff:ff:ff
Data: Who is the MAC-Gateway
for this network?

If there is a gateway on the network, the gateway sends a message containing its serial number back to your computer.
From: 98:2f:4e:78:c1:b4
To: 0f:2a:b3:1f:b3:1a
Data: I am the gateway
Welcome to my network

If there are no replies, your computer waits a few seconds and then assumes there is no gateway for this network. When there is no gateway, your computer might show a different WiFi icon or not show the WiFi icon at all. Sometimes there can be more than one gateway, but we will ignore that for a while because it is a little complex and not very common.
Once your computer receives a message with the MAC address of the gateway, it can use that address to send packets that it wants the gateway to forward to the Internet. From that point on, all of your computer’s packets have the actual serial number of the destination. You want to use the broadcast address as little as possible because every computer connected to the WiFi receives and processes any messages sent to the broadcast address to make sure the messages were not intended for them.

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