configuring access for internal interface only in cisco

I currently have tested a new router which is to be used for IPSec and consequently has a real world adddress on one side of the router - i want to be able to login via telnet from internal IP's but i can also log in via the real world address - what is the easiest way to restrict this access and olny allow internal networks to go via the LAN address?
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access-list applied to the vty lines
Assume: internal LAN =
Assume: real world IP =

access-list 6 permit
access-list 6 permit
line vty 0 4
 access-class 6 in

! Done

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