Telnetting

If you want to allow only certain IP addresses to use Telnet to access the router, you must use the access-class command. The command access-class nn in defines an access list (from 1 through 99) that allows access to the virtual terminal lines on the router. The following configuration commands allow incoming Telnet access to the router only from hosts on network 192.85.55.0:

access-list 12 permit 192.85.55.0 0.0.0.255
line vty 0 4
access-class 12 in
conf t
line vty 0 5
login
password xxxx (where the x's are your password)

You may want to look over the Cisco explanation papers:
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_fix/826/swg/routconf.htm#wp1015102

If your using a 1700 series access router, then you probably already have access lists created.

I would password your router at both levels so that "if" someone would be able to get the IP address of the router, they would have to have a password to connect and then another one [different password to enter global commands.

Depending upon the complexity of your network, I would also change your SMNP private RW to something only you know.

One of the biggest headaches with Cisco routers. One of the reasons is that there is an implied deny at the end of the list [meaning if you create an access list and do not specifically allow, communications not meeting the access list parameters is denied even though deny doesn't show in the access list]. Creating a bunch of access lists just makes more headaches in trouble shooting should you have a problem with the access router.

A simple method to secure your router is via passwording.