In short, you need a device that supports Network Address Translation (NAT). Almost all routers made today support NAT, as PriceD suggests.
The internal network addresses PriceD suggests you can use are mostly inaccurate.
The ENTIRE 192.x.x.x block is NOT available for NAT addressing - only the addresses starting with 192.168 are available - 192.168.x.x
The ENTIRE 10.x.x.x block IS available (PriceD is correct about this one)
The ENTIRE 172.x.x.x block is NOT available - ONLY addresses that start with 172.16 through 31.x.x are available.
For more information, please see:
http://en.wikipedia.org/wiki/Network_address_translationand RFC1918 -
http://tools.ietf.org/html/rfc1918