Every device in your home has its own address, but the internet only ever sees a single address for the whole house: the one your router holds on its outside interface. Network address translation is the trick that makes many devices look like one.
As traffic leaves, the router rewrites the source address to its public one and records which internal device and port the connection belongs to. When a reply arrives it is matched against that record and rewritten again on the way back in. Nothing outside knows how many devices are behind the router, because the translation table is the only thing that does.
Three things to remember
- Private addresses are not routable across the internet, so the router substitutes its public one.
- The router keeps a translation table so that replies can be matched back to the right device.
- A connection started from outside has no entry to match, which is why inbound access needs to be configured deliberately.
Related explanations
- What Does 255.255.255.0 Actually Do? (It’s Been Hiding in Plain Sight for Years)
- What Happens When You Hit Send? (Your Download Is Faking It)
New explanations go up on the channel first.
Like the drawings? Stick Figure Internet merch.
