Hi,
cisco 881-k9 comes with 1 wan port fast ethernet 4 and 4 integrated switch ports fast ethernet port from 0 to 3.
you cannot directly use switch ports as layer 3 ports and assign IP address.
but you can use switch port as layer 3 port by creating a vlan and assign the port to the vlan.
interface vlan 10
desc " Etherswitch wan interface"
ip address <<ip address>> <<subnet mask>>
interface FastEthernet1
switchport access vlan 10
no ip address
for more information please go through
http://www.cisco.com/c/en/us/products/collateral/routers/1800-series-integrated-services-routers-isr/prod_white_paper0900aecd8064c9f4.html
