A host communicates with a gateway, a DHCP server or a PPP peer of ISP to determine an IPv6 address, also receives a public key certificate from the gateway, the DHCP server or the PPP peer of ISP, and sends a public key certificate including an IPv6 address to a communication counterpart. The host receives a new public key certificate from the gateway, the DHCP server or the PPP peer of ISP when necessary.