Data transmissions

IPv4 uses 32 bits, written as four numbers, giving about 4 billion addresses — which ran out. IPv6 uses 128 bits, written as groups of hex digits, so the supply is effectively unlimited.

A public address is unique across the whole internet and reachable from anywhere on it, which makes it scarce. A private address is used inside one network only and is not routable on the internet, so millions of networks can reuse the same one.

The router swaps each device's private address for the organisation's one public address on the way out, remembers which device asked, and swaps it back on the reply. It exists because public IPv4 addresses are scarce.

A static address never changes, which anything that must always be findable needs — a web server, for instance. A dynamic address is assigned by DHCP when a device joins and may differ next time, which suits laptops and phones and reuses addresses efficiently.

Private addresses are not routable on the internet, so nothing outside can address an internal machine directly — it can only reply to a conversation that machine started. A firewall is still needed as well.

Twisted pair sends bits as electricity down copper, fibre optic sends them as light down glass, and wireless sends them as radio through the air.

Attenuation is the signal getting weaker with distance — copper fades after about 100 m, fibre runs kilometres. Interference is something else corrupting the signal: electrical noise for copper, walls and other radios for wireless.

Between buildings its very low attenuation and immunity to interference are worth paying for. To a desk the run is short and the bandwidth is never used, so copper's much lower cost wins across hundreds of sockets.

The signal leaves the building, so anyone nearby can receive it, whereas tapping copper or fibre requires physical access to the cable. That is why wireless must always be encrypted.

It is shared with every device nearby, and walls, appliances and other networks all cause interference, so the rate a device actually achieves varies with conditions rather than being fixed.

Data is cut into small packets, each sent independently across the network and joined back together at the destination. No line is reserved for a whole transfer, so many conversations share the network at once.

The destination address, so routers know where to send it; the source address, so a reply or a request to resend can be sent back; and a sequence number saying where this piece belongs in the message.

Each packet is routed independently, and different routes take different times. This is expected behaviour, which is exactly why sequence numbers are in the header.

A switch delivers a frame to the one device it is addressed to within a single network. A router reads the destination IP address and chooses the next network to forward the packet to, one hop at a time.

A reserved line is wasted whenever nobody is talking, and a break in it ends the conversation. Packet switching fills the gaps with other traffic and routes around breaks, and a lost packet means resending one packet rather than everything.