Mustafa

Yes you can assign a specific IP address to a specific mac-address. See this attached link for details:

http://www.cisco.com/en/US/docs/ios/12_4t/ip_addr/configuration/guide/htdhcpsv.html#wp1074511

Once you have done this you can then use an access-list to block specific IP addresses ie. lets say you want to block host 192.168.5.10

access-list 101 deny ip host 192.168.5.10 any

access-list 101 permit ip any any

You can modify the above acl to be more specific ie. TCP ports if needed.

You then apply the acl inbound on the LAN interface of your 877

int fa0/1

ip access-group 101 in

Jon