A method for setting a mode of a server includes setting the mode of the server to the negative mode when the sever boots, monitoring the network to detect a DHCP offer packet provided by another DHCP server in the network, setting the mode of the server to the positive mode when no DHCP offer packet is provided by the another DHCP server in the network and analyzing the DHCP offer packet to determine the mode of the another DHCP server when the DHCP offer packet is provided by the another DHCP server in the network. The modes of the server can be set automatically according to a state of a network in which the server works.