The IPv4 Address attribute specifies an IPv4 network address of the responder. This attribute is optional; implementations SHOULD include it in Hello frames if they have an IPv4 address.
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IPv4 Address
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Type (1 byte): This field MUST be set to 0x07.
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Length (1 byte): This field MUST be set to 0x04.
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IPv4 Address (4 bytes): This field specifies an IPv4 address of the responder. This field's value MUST be an address of the network interface over which the frame is sent, if it has an IPv4 address. If there are multiple IPv4 addresses on the network interface, the device is free to choose any one of them. If an IPv4 address is not available on the network interface over which the frame is sent, the device MAY use an IPv4 address on a different network interface. However, if the responder sets the MW bit in the Characteristics attribute, the address MUST be one which is reachable via the interface over which the frame is sent (for example, if the device is a router), and if there is no such address, the responder MUST NOT include the IPv4 Address attribute.