Sometime you may like to limit only certain IP address to access your Cisco router / switch by using telnet. You may implement it by configuring the access list and apply it to the line vty 0 4 line.
access-list 5 permit 10.0.0.1
access-list 5 permit 10.0.0.2
access-list 5 permit 192.168.3.1
access-list 5 permit 192.168.0.1
access-list 5 deny any
!
line con 0
line vty 0 4
access-class 5 in
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