An ISP wants to aggregate four customer networks into a single route advertisement. The customers hold: 172.16.4.0/24, 172.16.5.0/24, 172.16.6.0/24, and 172.16.7.0/24. Which prefix correctly summarizes all four networks?
A172.16.0.0/22 — a /22 starting at the class B base address
B172.16.4.0/22 — a /22 starting at the first network address
C172.16.4.0/21 — a /21 covering eight /24 networks
D172.16.0.0/16 — the full class B block containing all four networks
172.16.4.0 in binary has third-octet 00000100; 172.16.7.0 has 00000111. The first 22 bits are identical across all four networks (the leading bits 000001 are constant in the third octet; only the last two bits vary from 00 to 11). So the aggregate is 172.16.4.0/22. Option A (172.16.0.0/22) covers 172.16.0.0–172.16.3.0, not the target networks. Option C (/21 from 172.16.4.0) is not /21-aligned — a valid /21 from 172.16.0.0 covers 172.16.0.0–172.16.7.0, eight networks including ones not belonging to this ISP. Option D is technically inclusive but far too broad.
Question 2 Multiple Choice
A network operator aggregates routes for 10.0.0.0/24 through 10.0.7.0/24 into a single advertisement of 10.0.0.0/21. One subnet, 10.0.5.0/24, is later decommissioned and becomes unreachable. The operator continues advertising the aggregate. What is the likely result?
ARouters automatically detect the unreachable subnet and stop forwarding traffic to it
BTraffic destined for addresses in 10.0.5.0/24 is attracted by the aggregate route and then dropped, creating a routing black hole
CThe aggregate becomes invalid and is withdrawn by BGP automatically
DTraffic to 10.0.5.0/24 is rerouted through alternative paths within the aggregate
This is the routing black hole problem. The aggregate 10.0.0.0/21 promises reachability for all 2048 addresses in that block. External routers forward traffic to the originating AS based on this advertisement. But inside the AS, 10.0.5.0/24 no longer exists — there is no specific route, and the traffic is dropped. The aggregate attracts the traffic but cannot deliver it. Solutions include withdrawing the aggregate, advertising a more-specific null route for the decommissioned prefix, or never advertising an aggregate without a specific backing route for every covered subnet.
Question 3 True / False
Route aggregation reduces routing table size, which decreases the memory and lookup time required by routers handling Internet-scale traffic.
TTrue
FFalse
Answer: True
True. This is the primary motivation for aggregation. Without it, each individual network would require a separate routing table entry — the global Internet routing table would contain billions of entries (one per customer network). Aggregation allows ISPs to advertise summary routes covering hundreds or thousands of customer networks in a single entry. The global BGP table currently holds over a million entries even with aggressive aggregation; without it, routing infrastructure would be untenable. Reduced table size also speeds up longest-prefix-match lookups, reducing forwarding latency.
Question 4 True / False
Any two numerically adjacent /24 networks (e.g., 10.0.1.0/24 and 10.0.2.0/24) can usually be aggregated into a single /23 prefix.
TTrue
FFalse
Answer: False
False — adjacency is necessary but not sufficient; alignment is also required. A valid /23 must start on a /23 boundary (an address where bit 23 is 0 in the third octet). 10.0.0.0/24 and 10.0.1.0/24 are properly aligned and aggregate to 10.0.0.0/23. But 10.0.1.0/24 and 10.0.2.0/24 cannot form a valid /23 together: no single /23 covers exactly these two; any /23 covering 10.0.1.0 would cover 10.0.0.0, while any /23 covering 10.0.2.0 would cover 10.0.3.0. Alignment ensures the aggregate is a well-defined CIDR block.
Question 5 Short Answer
Why does route aggregation require that component networks be both contiguous and properly aligned on a CIDR boundary, rather than simply contiguous?
Think about your answer, then reveal below.
Model answer: A CIDR prefix describes a power-of-two block starting at an address divisible by its block size. Contiguity alone is insufficient because an arbitrary sequence of networks may not start on a valid CIDR boundary. For example, 10.0.1.0/24 and 10.0.2.0/24 are contiguous, but no valid /23 covers exactly these two — a /23 at 10.0.0.0 also includes 10.0.0.0/24, and a /23 at 10.0.2.0 also includes 10.0.3.0/24. Alignment ensures the aggregate prefix describes exactly the intended block without accidentally advertising reachability for addresses outside the aggregated range.
CIDR prefixes have a mathematical structure: a /n prefix covers a block of 2^(32-n) addresses starting at an address divisible by 2^(32-n). This power-of-two alignment means aggregation only works cleanly when networks were originally allocated from contiguous, aligned blocks — which is why IP address planning matters so much. ISPs that allocate from aligned blocks can aggregate cleanly; fragmented allocations resist aggregation and contribute to routing table bloat, which is a real and growing problem for Internet infrastructure.