An Internet Protocol, IP, address allocation method performed by a relay device, the method comprising the following steps: a.) receiving from a Dynamic Host Configuration Protocol, DHCP, server, a first IP address allocated to a first user (S 501); b.) detecting that the first IP address allocated to the first user and an IP address already allocated by the DHCP server to a second user are duplicate, and that the IP address already allocated to the second user is still within a lease time, wherein the relay device stores the IP address already allocated to the second user and the lease time of the IP address already allocated to the second user (S 502); c.) sending a force renew message to the second user, wherein the force renew message instructs the second user to request IP address renewal, and the force renew message carries an IP address of the DHCP server (S 503); d.) waiting for a predetermined time, and determining that no DHCP renew request message is received from the second user within the predetermined time, wherein the DHCP renew request message is used to request to renew the IP address of the second user (S 504); e.) sending a DHCP reject message to the DHCP server, wherein the DHCP reject message is used to notify the DHCP server that the first IP address allocated to the first user is unavailable, and the DHCP reject message carries the first IP address allocated to the first user and a Media Access Control, MAC, address of the first user (S 508); f.) interacting with the first user and the DHCP server to reallocate a second IP address to the first user, wherein the second IP address reallocated to the first user is different from the IP address already allocated to the second user (S 509). a.) receiving from a Dynamic Host Configuration Protocol, DHCP, server, a first IP address allocated to a first user (S 501); b.) detecting that the first IP address allocated to the first user and an IP address already allocated by the DHCP server to a second user are duplicate, and that the IP address already allocated to the second user is still within a lease time, wherein the relay device stores the IP address already allocated to the second user and the lease time of the IP address already allocated to the second user (S 502); c.) sending a force renew message to the second user, wherein the force renew message instructs the second user to request IP address renewal, and the force renew message carries an IP address of the DHCP server (S 503); d.) waiting for a predetermined time, and determining that no DHCP renew request message is received from the second user within the predetermined time, wherein the DHCP renew request message is used to request to renew the IP address of the second user (S 504); e.) sending a DHCP reject message to the DHCP server, wherein the DHCP reject message is used to notify the DHCP server that the first IP address allocated to the first user is unavailable, and the DHCP reject message carries the first IP address allocated to the first user and a Media Access Control, MAC, address of the first user (S 508); f.) interacting with the first user and the DHCP server to reallocate a second IP address to the first user, wherein the second IP address reallocated to the first user is different from the IP address already allocated to the second user (S 509).