GATE CS 2026 Set 1 Q46 — Routing Protocols: Shortest Path, Flooding, Distance Vector, Link State

To determine which address blocks can be assigned, we analyze the ISP's block and the client's requirements:

1. ISP Address Range:
The ISP has the block $202.16.0.0/15$. The first $15$ bits are fixed, which means the second octet can range from $16$ ($00010000_2$) to $17$ ($00010001_2$). Thus, any assigned block must fall within the range $202.16.0.0$ to $202.17.255.255$.

2. Client Block Size:
The client requires $6000$ IP addresses. The smallest power of $2$ that can accommodate this is $2^{13} = 8192$. This corresponds to a prefix length of: $$32 - 13 = 19$$

3. Valid $/19$ Block Boundaries:
For a block $202.Y.Z.0/19$ to be valid:

• The second octet $Y$ must be $16$ or $17$.
• The first $3$ bits of the third octet $Z$ belong to the network prefix, and the remaining $5$ bits must be $0$ for the network address. Thus, $Z$ must be a multiple of $2^5 = 32$ (i.e., $0, 32, 64, 96, \dots$).

Evaluating the Options:

• A. $202.16.0.0/19$: $Y = 16$ (valid) and $Z = 0$ (multiple of $32$, valid).
• B. $202.17.64.0/19$: $Y = 17$ (valid) and $Z = 64$ (multiple of $32$, valid).
• C. $202.16.32.0/19$: $Y = 16$ (valid) and $Z = 32$ (multiple of $32$, valid).
• D. $202.17.24.0/19$: $Y = 17$ (valid), but $Z = 24$ is not a multiple of $32$, making it an invalid network boundary.