Sharing one public address

A small office has 30 laptops, a few phones and two printers. The broadband provider gives it one address on the internet.

Yet every laptop browses at the same time, and each reply reaches the right desk. The router does this by keeping notes. In this lesson you read those notes.

Three ranges of addresses are kept for private use. Any office or home can use them, so thousands of networks use the same ones.

That is fine because they never travel across the internet. Only the router's public address does. If you see 192.168. or 10. at the start, you are looking at a private address.

def is_private(address):
    # The three private ranges: 10.x.x.x, 172.16.x.x to 172.31.x.x, 192.168.x.x
    parts = address.split(".")
    first = int(parts[0])
    second = int(parts[1])
    if first == 10:
        return True
    if first == 172 and second >= 16 and second <= 31:
        return True
    if first == 192 and second == 168:
        return True
    return False


for address in ["192.168.1.23", "10.0.4.7", "203.0.113.10", "172.20.0.5"]:
    print(address, "private:", is_private(address))

# Add "172.32.0.5" to the list. Is it private?

Your laptop can reach devices on its own network directly. For anything else, it sends the traffic to one router: the default gateway.

Below is real output from ip route on a Linux laptop. The first line names the gateway. The second says which addresses are local. If the gateway is wrong or down, the laptop can still print, but cannot reach any website.

$ ip route
default via 192.168.1.1 dev wlan0
192.168.1.0/24 dev wlan0 proto kernel scope link src 192.168.1.23

When a laptop sends a request out, the router swaps the laptop's private address for its public one. It also picks a port, and writes a line in its NAT table.

When the reply comes back to that port, the router reads the line. It swaps the address back and sends the reply to the right laptop. A reply with no line in the table is dropped.