The present disclosure relates to the mobile communication field and discloses a prefix allocation method, a network system, and a Local Mobility Anchor (LMA). With the prefix allocation method, the network system and the LMA provided in the present disclosure, the problem that a shared prefix cannot be allocated to a multi-interface (IF) Mobile Node (MN) is solved. The prefix allocation method includes: receiving a registration request for a second IF of the MN from a Mobile Access Gateway (MAG); obtaining a first Home Network Prefix (HNP) that is already allocated to a first interface (IF) of the MN; and allocating the first HNP shared with the first IF to the second IF. The LMA obtains the first HNP that is already allocated to the first IF and allocates the first HNP to the second IF. In this way, a shared prefix is allocated to the multi-IF enabled MN.