I want to add layer 3 router interface on C2851. I did a search and find the following part number"
Four port 10/100 Ethernet switch interface card
My quesions is:
Will the 4 ethernet ports share just one layer 3 interface or they are 4 seperate layer 3 interfaces?
The 4 Ethernet ports in the HWIC-4ESW are layer 2 ports. You can put all 4 ports into a single VLAN and have a single layer 3 interface if you want. Or you can put each port into a separate VLAN, define 4 VLAN interfaces, and therefore have 4 layer 3 interfaces.
It is quite flexible and the results will reflect what you want it to do.