Disclosed are various embodiments for managing network address allocations using prefix allocation trees. In one embodiment, a request is received to allocate a network address block. The request specifying a prefix size for the network address block. A particular network address block having the prefix size that is allocatable is determined based at least in part on a prefix allocation tree representing the available and allocated network address space. The prefix allocation tree is updated to indicate that the particular network address block is allocated. An identification of the particular network address block is returned in response to the request to allocate the network address block.