As per RFC 2865
This Attribute indicates the physical port number of the NAS which is authenticating the user. It is only used in Access-Request packets. Note that this is using "port" in its sense of a physical connection on the NAS, not in the sense of a TCP or UDP port number. Either NAS-Port or NAS-Port-Type (61) or both SHOULD be present in an Access-Request packet, if the NAS differentiates among its ports.
A summary of the NAS-Port Attribute format is shown below. The fields are transmitted from left to right.
Type (IETF Attribute Number) - 5 for NAS-Port.
Length - 6
Value - The Value field is four octets.