Your modem is connected to your four port router, your iMac is connected to your four port router, your ABS is connected to your four port router, and your iBook is connected wirelessly to your ABS. As you appear to recognize the problem is that your iMac and iBook are now on different subnets. In other words, the third series of IP digits needs to match one another, such as 10.0.0.1 and 10.0.0.2. You need to configure either the router or the ABS so that their LAN IPs have matching subnets.