A method for allowing a device in an external network to access a server in an internal network via a public IP address. A routing equipment receives a message carrying a destination IP address and a destination port number from the device, matches the destination IP address and the destination port number with external characteristics of the server in a mapping table established on the routing equipment. If a match exists, the routing equipment replaces the destination IP address and the destination port number with the internal IP address and the internal port number. The mapping table includes a correlation between external characteristics and internal characteristics of the server. The external characteristics include the external IP address and the external port number of the server. The internal characteristics include the internal IP address and the internal port number.