Private IP addresses

Global public IP addresses are limited, simply because there are only so many 32 bit numbers! As such they are managed carefully, and networks that do not need to have every machine addressable from the public internet do not need global public IP addresses. As such a number of addresses have been allocated for use in private networks. These private IP addresses will never be allocated to anyone, and so their use can never clash with publicly available addresses. Private addressed networks can still communicate with machines on the internet, but only via address translation or proxy as the private addresses are not valid on the public internet.

If you make a private network and need addresses, you must only ever use these reserved private addresses, otherwise the address you have picked may belong to someone, and you will effectively mask access to those addresses from your network.

Private IP address ranges are as shown below:

These reserved IP addresses are shown in: http://www.nthelp.com/40/ip.htm. The TCP/IP addresses reserved for 'private' networks are invalid addresses on the internet and routers don't route them.