open & close telnet

[Beavis]

hello,   i am a newbie, hi newbie's   i had a friend configure a linux box to act as a firewall, ip masquaring etc. it also is set up as a DHCP, spitting out IP's from 192.168.1.100-200   he made it so secure that i can't even get in, go figure. my telnet port is refusing connections, how do i open it again and also, is there any way to make linux only accept a telnet connection from a specific remote server?   thankx beavis   going from newbie to pro