Guest, the parameters for the machines are set by the administrator in charge. If Mudcat.org is put on the list of forbidden sites, it is closed for those on the network.

Here is the UK website for RM Safety Net: RM Filter

"RM Safety Net Plus works on a 'list' basis whereby the administrator can build up organized lists of permitted or denied websites."
"It is possible to operate the site as a "walled garden" if preferred. This would allow a school to prevent access to ALL internetr websites as standard and then permit access to just those sites the school considered to be appropriate."