A packet with the destination IP address $145.36.109.70$ arrives at a router whose routing table is shown. Which interface will the packet be forwarded to?

To determine the forwarding interface, we apply the Longest Prefix Match (LPM) algorithm for the destination IP address $145.36.109.70$.

First, convert the 3rd octet ($109$) to binary:

Now, test each entry in the routing table:

1. $145.36.0.0/16$ (Interface E1):

   • Requires matching the first $16$ bits ($145.36$).

   • The destination address has $145.36$, so it matches (prefix length = $16$).

2. $145.36.128.0/17$ (Interface E2):

   • Requires the first $17$ bits to match.

   • The 17th bit (most significant bit of the 3rd octet) must be $1$ ($128_{10} = 10000000_2$).

   • The destination's 17th bit is $0$. Hence, it does not match.

3. $145.36.64.0/18$ (Interface E3):

   • Requires the first $18$ bits to match.

   • The first two bits of the 3rd octet must be 01 ($64_{10} = 01000000_2$).

   • The destination's first two bits of the 3rd octet are 01. Hence, it matches (prefix length = $18$).

4. $145.36.255.0/24$ (Interface E4):

   • Requires the 3rd octet to be $255$ ($11111111_2$).

   • The destination's 3rd octet is $109$, so it does not match.

Comparing all matching entries, $145.36.64.0/18$ has the longest prefix length ($18 > 16$). Therefore, the router forwards the packet to interface E3 (Option A).