On
IRC, a
hostmask refers to all or part of the
dns entry for a certain user-- the domain name returned when
reverse-dnsing the user's IP adress. Hostmask can be found using
/whois.
Partial hostmasks are used in identifying certain users, either to ensure they really are who they claim to be (the X bot on undernet only lets people log in their accounts if they match the registered hostmask for that account) or, far more often, for bans. These hostmasks are defined using wildcard expansion; for example, if the user mcc has the hostmask adsl-207-192-181-159.dsl.hstntx.swbell.net, and you want to ensure mcc can never, ever get in the channel again, you place a ban on the *.dsl.hstntx.swbell.net hostmask, thus banning everyone whose hostmask ends with dsl.hstntx.swbell.net. Of course, this will probably apply to and thus wind up banning a bunch of innocent people who are not mcc, which is why you would want to be rather more specfic with your bans. See /mode