Multiple IP Adresses (simple question)

[Mc-queen]

Hi to everybody,

My question may have simple answer as well:

I have an isp that provide me 4 public ip adresses, but 1 modem. Now i have the modem, then a router (for a private lan purposes), but i want to configure almost 1 public ip more to make a dns system using 2 linux boxes and bind (using the 2 public ip´s).
More info: all ip´s are static - i want to inform these dns to my registrar to let me register (point) domains to my network/servers.

What is the best approach to do this? i need some hardware here i guess, cause i don´t know from where take the another ip if i have just one modem!

i´m missing something here.. any ideas? links? books to read?

Thanks in advance.

Kchow!