Inside your building (your router’s life)

Every number on it was measured on a real home network, with six devices home.

Before you start. You already met your router as the concierge in DNS Resolution, looking up a name and handing back an address, and as the first hop in How a parcel finds any house on earth, deciding which way a packet leaves the building. This lesson gives that same box two harder jobs: handing out addresses of its own, and covering for the world not having enough of them.

The idea in 30 seconds

Your router hands out flat numbers to every device that joins, over a lease with a clock, called DHCP - not a one-time gift.
One street address can cover a whole building because the doorman keeps a book swapping every reply’s address back to the right flat: NAT, the reason a global address shortage never reached your sofa.
A street address gets a letter to the right building; only a hardware doorplate, found by ARP and called a MAC address, gets it to the right device on the wire.
Two postal systems run through your router at once: the old one, stretched thin since 2011, and IPv6, wide enough that no cover story is needed.
None of the doorman’s messages (the lease offers and the “who has this flat number?” shouts) are signed, which is why the network you are on can be spoofed, not just overheard.

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