This command will enable the use of 'show ip cache flow' to monitor the IP traffic passing to the interface.

1. Enable the 'ip route-cache flow' to the interface to monitor using the ff command:

conf t
interface fa0/0
ip route-cache flow
end

2. Run the ff command in privilegde mode:

show ip cache flow

You should see somethind similar to this:

SrcIf         SrcIPaddress    DstIf         DstIPaddress    Pr SrcP DstP  Pkts
Fa0           63.81.202.130   Se0.500       64.78.17.15     06 08B6 01BB   565
Fa0           63.81.202.130   Se0.500       64.78.17.15     06 CC10 01BB   290
Fa0           63.81.202.130   Se0.500       64.78.17.15     06 CC07 01BB   439
Fa0           63.81.202.130   Se0.500       64.78.17.15     06 CC08 01BB  4050
Fa0           63.81.202.130   Se0.500       64.78.17.15     06 CC0F 01BB   332
Fa0           63.81.202.130   Se0.500       64.78.61.166    06 0FB8 01BB     1
Fa0           63.81.202.130   Se0.500       64.78.61.166    06 0FCA 01BB     1
Fa0           63.81.202.130   Se0.500       64.78.61.166    06 0FCD 01BB     1
Fa0           63.81.202.130   Se0.500       64.78.61.166    06 127C 01BB     1