4.6 A Different Kind of Address Reuse

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If you know how to find the IP address on your laptop, you can do a little experiment and look at the different IP addresses you get at different locations. If you made a list of the different addresses you received at the different locations, you might find that many of the locations give out addresses with a prefix of “192.168.”.

This seems to be a violation of the rule that the network number (IP address prefix) is tied to the place where the computer is connected to the Internet, but a different rule applies to addresses that start with “192.168.” (The prefix “10.” is also special).

Addresses that start with “192.168.” are called “non-routable” addresses. This means that they will never be used as real addresses that will route data across the core of the network. They can be used within a single local network, but not used on the global network.

So then how is it that your computer gets an address like “192.168.0.5” on your home network and it works perfectly well on the overall Internet?

This is because your home router/gateway/base station is doing something we call “Network Address Translation”, or “NAT”. The gateway has a single routable IP address that it is sharing across multiple workstations that are connected to the gateway. Your computer uses its non-routable address like “192.168.0.5” to send its packets, but as the packets move across the gateway, the gateway replaces the address with its actual routable address. When packets come back to your workstation, the router puts your workstation’s non-routable address back into the returning packets.

This approach allows us to conserve the real routable addresses and use the same non-routable addresses over and over for workstations that move from one network to another.

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