An address translation unit is provided for use in a computer system. The unit contains a set of page table entries for mapping from a virtual address to a packet address. Each page table entry corresponds to one page of virtual memory, and typically includes one or more specifiers. Each specifier relates to a different portion of the page, and maps from that portion of the page to a corresponding range of packet addresses. Accordingly, the unit allows for address translation to be performed with a sub-page granularity.