If you create one VLAN per physical switch port (the 881 seems to support up to 8 VLANs), you can give each VLAN a separate ip subnet. For example:
f0 has VLAN 100 and subnet 192.168.0.0/24
f1 has VLAN 101 and subnet 192.168.1.0/24
f2 has VLAN 102 and subnet 192.168.2.0/24
f3 has VLAN 103 and subnet 192.168.3.0/24
i.e. if a connection is made from 192.168.2.65
you will know it connected through VLAN 102
and interface f2
.
The syntax for assigning VLANs to interfaces is described here and seems to be:
Router(config)# Interface fastethernet2
Router(config-if)# switchport access vlan 102
Router(config-if)# no shutdown
Router(config-if)# end
The syntax for assigning an ip address to a VLAN interface (i.e. the default gateway address for the clients) is seen throughout the configuration guide and seems to be in configured form:
Interface Vlan102
ip address 192.168.2.254 255.255.255.0
More info on VLANs can be found here and on subnetting here.