User-level network interfaces allow applications direct access to the network without operating system intervention on every send and receive. Messages are transferred directly to and from user-space by the network interface while observing the traditional protection boundaries between processes. Current user-level network interfaces limit this message transfer to a per-process region of permanently-pinned physical memory to allow safe DMA. This approach is inflexible in that it requires data to be copied into and out of this memory region, and does not scale to a large number of processes. This paper presents an extension to the U-Net user-level network architecture (U-Net/MM) allowing messages to be transferred directly to and from any part of an application's address space. This is achieved by integrating a translation look-aside buffer into the network interface and coordinating its operation with the operating system's virtual memory subsystem. This mechanism allows network buffer pages to be pinned and unpinned dynamically. Two implementations of U-Net/MM are described, demonstrating that existing commodity hardware and commercial operating systems can efficiently support the architecture.