I have around 23 000 Contacts and I need to do Mass Update on them after selecting the criteria. Because of the pagination the checkbox next to the header selects only 100 records, as I'm assuming?

Shouldn't this checkbox be selecting all records that match the criteria? What if I have 10 000 records I want to change, I need to do 100 repetitions of my task, not really "mass" isn't it?