Blocking Someone From a Domain Once You Have Their IP Address
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A) Find The MAC Address from a given IP Address
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1) Open CiscoWorks
2) login 
3) Campus Manager
4) User Tracking 
3) Sort by IP Address by clicking the IPAddress column at the top. 
4) Scroll down and find the IP Address in question 
5) Note the MAC address to the left of the IP Address in question *
   * Verify the hostname as precaution if it was provided to you. 
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ALTERNATIVELY:
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Given the name of the machine:
a)c:\>ping -a machine_name    ---- typing this at the DOS prompt should resolve to the ip address
b)c:\>nbtstat -a ip_address   ---- gives the MAC Address & who is logged onto the machine currently
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B) The actual blocking Of The Machine
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6) Login to the core switch 
7) enable mode 
8) configure terminal
9) mac-address-table static 0000.0000.0000 vlan 1 drop *
   * Replace 0000.0000.0000 with the MAC address you got in step 5.