1. A method of classifying received packets in a security system, the method comprising: parsing a received packet and extracting a payload from the parsed packet; scanning the payload to check whether or not a predetermined signature code is included in the payload; if it is determined from the result of the scanning that the predetermined signature code is included in the payload, generating a presumptive signature based on information included in the predetermined signature code; and determining whether or not the generated presumptive signature is identical with a signature corresponding to the predetermined signature code, and allocating an classification identifier (ID) to the received packet according to the result of the determination, thereby classifying the received packet according to the classification ID, wherein the predetermined signature code is formed by a part of the signature corresponding to the signature code.