Most likely someone, using the same isp that you do, has been acting like a jerk and got the address banned.

Try e-mailing the Admin of the Mud, they are usually able and willing to allow a new player to create a char, even if the isp is newbanned.
There should be an e-mail address on their website that you can use.

Your best bet is to post a note on the administrator's . There have been quite a few people sitebanned over the years; DartMUD takes cheating quite seriously.