Remember you need two network interfaces to run a home network. In your case (dialup) one will be the modem and the other will be the ethernet connected to your hub/switch. Make sure you setup the modem interface (ppp0) as your ISP's settings and setup the ethernet interface (eth0) with an internal ip address such as "10.10.10.1". You don't need a tool to create and IP you just assign one to your interface. How do you know what IP addresses are available for internal networks? use this chart:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
The other IP blocks are all mostly assigned to large companies or corporations. Ford motor company has whole 019/8 (A) network for example, that like 16,777,216 IP addresses if my math is correct.
A assignments chart can be found here: