When advertising a summarized route in IPv4, which network address from the block is used as the summary (aggregate) address?

Introduction / Context:
This question tests your understanding of route summarization, also called route aggregation, in IPv4 networks. When you summarize multiple contiguous networks into a single advertisement, you must choose a single network address and prefix length to represent the entire block. Knowing which specific address is used as the summary is key to configuring correct static summaries and interpreting dynamic protocol summaries.

Given Data / Assumptions:
- We are considering a group of contiguous networks that can be summarized by a single prefix.
- The networks share a common set of high order bits and differ only in the host or lower order network bits.
- The question asks which of the networks in the block is used as the advertised summary address.
- Standard classless addressing and summarization rules apply.

Concept / Approach:
When summarizing, you examine the binary representations of the network addresses and identify the longest common prefix that covers all networks in the block. The summary address is created by taking that common prefix and setting all remaining bits to zero. This process always yields the numerically lowest network address in the range. Therefore, the summary address is the first network in the block, not the last or a middle one.

Step-by-Step Solution:
Step 1: Consider a simple example: summarizing 192.168.16.0/24 through 192.168.19.0/24 into a single route.Step 2: Convert the varying octet (the third octet) to binary: 16, 17, 18, and 19 correspond to 00010000, 00010001, 00010010, 00010011.Step 3: Find the longest sequence of common high order bits among these values; for example, they may share the first 6 bits, creating a /22 summary.Step 4: Construct the summary address by keeping the shared bits and setting all remaining bits to 0, which yields 192.168.16.0/22.Step 5: Observe that this summary network corresponds to the first network in the contiguous block (192.168.16.0), confirming that the first network is used as the summary address.

Verification / Alternative check:
Anytime you perform summarization by hand, you can confirm the result by listing the numerical range of addresses covered by the chosen summary. The lower bound of that range will always be the smallest network address in the group. If you attempted to use the last network as the summary base, the binary representation with zeros in the host bits would usually fall below the range you intend to cover, or with different prefix lengths would fail to encapsulate the entire block cleanly.

Why Other Options Are Wrong:
The last network address in the block does not become the summary base, because setting host bits to zero on that address will either overshoot or misalign the summary range. The next to last and second network options are arbitrary and do not follow the binary common-prefix rule. The idea of using any odd-numbered network is incorrect and has no basis in summarization algorithms.

Common Pitfalls:
One common error is to confuse the highest network in the block with the summary address because it feels intuitive to describe summaries from the top down. Another is to skip the binary analysis and rely on mental shortcuts that fail when the block size is not a power of two aligned on convenient boundaries. Always remember that summary addresses are based on common high order bits, which naturally produces the numerically lowest network in the range.

Final Answer:
The summary route uses the first network address in the block as the advertised summary address.