Abstract: In a method and system of establishing mobile network communications in a network such as a mobile IP network, an agent node receives a registration request message that includes routing address fields such as a home address field, a home agent address field, and a care-of address field. The registration request further includes an extension field. The extension field of the registration request message includes at least one substitute address value. The agent node interprets the registration request message by selecting addresses from the message. When an address value in an address field is a void value, the agent node selects a substitute address value from the extension field. When an address value in the address field is not a void value, the agent node selects the address value in the address field. Using the selected addresses, the agent node establishes a mobile network session with the mobile node.