It depends how your router is configured, really.

It's possible for a router to be configured to treat traffic originating outside the LAN much more strictly.
For example, you might have an "access list" which tells your router to refuse any telnet connection request originating from an IP outside your LAN, but to allow telnet sessions to be made between LAN hosts without complaint.
This would make it appear that you are vulnerable to telnet attacks when in fact your router protects against them. (Not that you should rest easy - more security is better!)

If you have a dinky Linksys router like most home users, it should be fairly easy to figure out how it is configured via the web-based GUI.
If you have a full-blown command line configurable router, well, you've got a lot to learn. If you let us know what router you're using, maybe we can help further.