An IP router implementing Classless Inter-domain Routing (CIDR) receives a packet with address 131.23.151.76. The router’s routing table has the following entries: Refer to the table below. The identifier of the output interface on which this packet will be forwarded is _____.

To determine the forwarding interface, we convert the destination address into binary and apply the Longest Prefix Match rule.

Destination IP: $131.23.151.76$

• First octet: $131 = 10000011_2$

• Second octet: $23 = 00010111_2$

Now, check each routing table prefix:

1. $131.16.0.0/12$:

   • Requires matching the first 12 bits.

   • $16 = 00010000_2$, so the first 4 bits of the second octet are $0001_2$.

   • The first 4 bits of $23$ are also $0001_2$.

   • Matches (length = 12).

2. $131.28.0.0/14$:

   • Requires matching the first 14 bits.

   • $28 = 00011100_2$, so the first 6 bits of the second octet are $000111_2$.

   • The first 6 bits of $23$ are $000101_2$.

   • No match ($000111_2 \neq 000101_2$).

3. $131.19.0.0/16$:

   • Requires matching the first 16 bits.

   • Second octet is $19 = 00010011_2$, which does not match $23$.

   • No match.

4. $131.22.0.0/15$:

   • Requires matching the first 15 bits.

   • $22 = 00010110_2$, so the first 7 bits of the second octet are $0001011_2$.

   • The first 7 bits of $23$ are $0001011_2$.

   • Matches (length = 15).

Among all matching entries ($/12$ and $/15$), the longest prefix is $/15$, which corresponds to output interface identifier 1.