N10-009 Question 120
Single answerIPv4 address classes: Class A, Class B, Class C, Class D, Class EA network administrator is reviewing firewall logs and notices traffic being sent to 239.1.1.1 during a video distribution test. A junior technician says the address must be a normal unicast host address because it is not in the company's private IP ranges. Based on IPv4 address classes, how should the administrator classify 239.1.1.1?
- A
Class C unicast address
- B
Class D multicast address
- C
Class E experimental address
- D
Class A public address
Show answer and explanation
Correct answer: B
Explanation
In the traditional IPv4 classful addressing model, Class A uses first octets 1-126, Class B uses 128-191, Class C uses 192-223, Class D uses 224-239 for multicast, and Class E uses 240-255 for experimental or reserved use. Therefore, 239.1.1.1 is a Class D multicast address, not a unicast host address. This matters operationally because multicast traffic is handled differently by switches, routers, ACLs, and firewall policies than standard unicast traffic. RFC 1112 defines IP multicasting for IPv4, and IANA documents identify 224.0.0.0/4 as the IPv4 multicast space.
- A. Incorrect.
Incorrect. Class C addresses use a first octet range of 192 through 223. Although 239.1.1.1 is close numerically to that range, 239 falls outside Class C. Class C addresses are traditionally unicast ranges, while 239.x.x.x is not.
- B. Correct.
Correct. Class D addresses use a first octet range of 224 through 239 and are reserved for multicast. Because 239.1.1.1 starts with 239, it is a Class D address. In practice, multicast traffic is commonly used for streaming, discovery, and one-to-many communication, which matches the video distribution scenario.
- C. Incorrect.
Incorrect. Class E addresses use a first octet range of 240 through 255 and are reserved for experimental purposes. A common mistake is assuming any non-unicast-looking address near the top of the IPv4 range is Class E, but 239 is still within the Class D multicast block.
- D. Incorrect.
Incorrect. Class A addresses use a first octet range of 1 through 126 in the traditional classful model. Since 239 is far outside that range, this cannot be a Class A address. The technician's assumption that any non-private address is a normal public unicast address is the misunderstanding being tested.