non routable ip address!

Class A: 10.x.x.x
Class B: 172.16.x.x - 17.31.x.x
Class C: 192.168.x.x

Above ip addresses are called "non-routable ip addresses". What does this mean? What is the meaning of non-routable?

Why using these ip addresses behind firwall?

Thank a lot!
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These address ranges are RESERVED for use by private network and any proper private <--> Internet router should be setup to block them from passing.

These are design so that you and many other site can freely use these addresses internally without worrying about conflicting with a real IP address on the internet.  Since your router will not pass them to the outside world, you don't have to worry about these conflicts or about getting true IP addresses for all your internal hosts.

If you are using a firewall or proxy, that box will take care of either performing the network request on behalf of the internal machine or will modify it's IP address (masquerade) to a proper one before sending the request to the internet.

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Tim HolmanCommented:
They're still routable, but only on internal networks.
qpsAuthor Commented:
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