problem with NiosII IIS

I add a IP core(from Mentor Graphics) to SOPC . The Nios II ISS generates the following warning message when I try to run it:

warning : sopc builder system component mentor_inventra_avalon_10_100_ethernet_m

ac_0 is not supported by the simulator. simulation may be incorrect if your soft

ware attempts to access it

warning : sopc builder system component mentor_inventra_avalon_10_100_ethernet_m

ac_1 is not supported by the simulator. simulation may be incorrect if your soft

ware attempts to access it

I've been hearded that iss supports simulation of the following peripherals:

. memory (ram/rom)

. avalon timer

. avalon jtag uart

. avalon uart

Accesses to unsupported peripherals will be ignored.

How can I solve this problem? Or, I should choose another way to begin writing and debugging Nios II applications before the final hardware configuration is determined?

Thanks!