Why does a Layer 3 device perform the ANDing process on a destination IPv4 address and subnet mask?
A to identify the broadcast address of the destination network
B to identify the host address of the destination host
C to identify faulty frames
D to identify the network address of the destination network.
The correct answer and explanation is :
The correct answer is:
D: to identify the network address of the destination network.
Explanation:
A Layer 3 device, such as a router, uses the ANDing process on the destination IPv4 address and the subnet mask to determine the network address of the destination. This step is crucial in routing and forwarding decisions.
What is ANDing in this context?
ANDing is a bitwise operation where each bit of the destination IP address is logically ANDed with the corresponding bit of the subnet mask. The subnet mask is a binary pattern that has a series of consecutive 1s followed by consecutive 0s (e.g., 255.255.255.0 corresponds to 11111111.11111111.11111111.00000000 in binary).
- When you AND the IP address with the subnet mask, the result isolates the network portion of the IP address.
- The bits corresponding to the 1s in the subnet mask preserve the network part of the IP address.
- The bits corresponding to the 0s in the subnet mask are zeroed out, removing the host portion.
Why is this important?
- Routing decisions: Routers do not forward packets based on the full IP address alone. Instead, they forward packets based on the destination network address. By extracting the network address from the destination IP, routers determine which interface or next-hop device the packet should be sent to.
- Subnetting: IP networks are divided into subnets to improve organization and efficiency. Identifying the correct network address ensures that traffic is routed within or between subnets correctly.
- Efficient forwarding: Without this process, routers would have difficulty matching IP addresses to the correct routing table entries because routing tables list networks, not individual host addresses.
Why not the other options?
- A (broadcast address): The broadcast address is found by setting all host bits to 1, not by ANDing with the subnet mask.
- B (host address): The host portion is the part zeroed out by ANDing. ANDing identifies the network, not the host.
- C (faulty frames): Faulty frames are identified by error-checking mechanisms at the Data Link Layer (Layer 2), not by ANDing IP addresses.
In summary, the ANDing operation helps the router to determine the network portion of the destination IP, which is essential for making proper routing decisions and forwarding packets correctly.