A method for releasing an internet protocol (IP) address by a packet data network gateway (PGW) in a mobile communication system comprises allocating, by the PGW, an internet protocol version 4 (IPv4) address and an internet protocol version 6 (IPv6) address to a user equipment (UE) to activate a packet data network (PDN) connection between the UE and the PGW. Receiving, by the PGW, a dynamic host configuration protocol (DHCP) release message from a serving gateway (SGW). Transmitting, by the PGW, to the SGW, a binding revocation message including information indicating release of the IPv4 address with the PDN connection between the UE and the PGW remaining active. Receiving, by the PGW, a binding revocation acknowledge message to acknowledge release of the IPv4 address from the SGW.