The error is caused because of the way that the quiz settings database is set up by default. The subnet field is declared to have a maximum length of 255 characters. The limit is arbitrary and can be changed using an ALTER TABLE command if you have a database admin capable of doing that, but it is a little dangerous to do on a production system.
However normally 255 should be more than sufficient on a reasonable network configuration. You should not be listing individual ip numbers, but one would expect that you would have a range of numbers allocated to the testing rooms that should be able to be described concisely with one of the options in the Quiz settings documentation.