What would the command(s) be to allow Telnet access from only 1 IP?

We have a problem where our replacement firewall is showing the MAC address on a 'show arp' command still show as "Incomplete". I've tried changing the main switch the network goes into with no luck.I can't seem to get the syntax correct on the arp c...

We're trying to install a new Wathguard Firebox Edge and when we replace our old Firebox SOHO, and login to the 1602R, "show arp" has question marks on the MAC address next to the IP of the Edge firewall. We've tried rebooting the router and running ...