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Wednesday, January 20, 2010

Day 16: Private Networks and NAT

A device directly connected to the Internet has a public IP address, this is routable. The number of public IP addresses is limited so RFC 1918 reserves class A, B and C networks for private use on an internal network. These addresses can be reused for multiple internal networks because the networks are not visible to the Internet or each other.

I've mentioned them a few times but once again a quick overview:
  • one class A address for private networks:, >16 million private addresses
  • 16 class B networks for private use: to, each networks allows > 65000 private addresses
  • 256 class C networks for private use: to, each network allows up to 254 private addresses
A router running NAT and PAT can allow devices on a private network to share a single public IP address and communicate over the Internet. Devices on a private network behind a router running NAT are not directly accessible on the Internet, providing additional security.

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