An ACL is applied inbound on a router interface. The ACL consists of a single entry: If a packet with a source address of 172.18.20.14, a destination address of 172.18.20.40, and a protocol of 21 is received on the interface, is the packet permitted or denied?

Access Control Lists (ACLs) operate by matching packet information against entries in the ACL. If a packet matches an entry, the corresponding action (permit or deny) is taken. If no match is found, the implicit default is to deny the packet.

The ACL entry provided is as follows:

• access-list 210 permit tcp 172.18.20.0 0.0.0.31 172.18.20.32 0.0.0.31 eq ftp

This entry specifies the following conditions for permitting traffic:

• Protocol: TCP
• Source IP address: 172.18.20.0/27, which translates to the range 172.18.20.0 to 172.18.20.31
• Destination IP address: 172.18.20.32/27, which translates to the range 172.18.20.32 to 172.18.20.63
• Destination port: FTP (port 21)

The packet in question has:

• Protocol: TCP (since FTP uses TCP and the protocol number for FTP control is 21)
• Source IP address: 172.18.20.14 (which falls within the source range 172.18.20.0 to 172.18.20.31)
• Destination IP address: 172.18.20.40 (which falls within the destination range 172.18.20.32 to 172.18.20.63)
• Destination port: 21 (which matches the ‘eq ftp’ condition)

Since the packet matches all the specified conditions in the ACL entry, it is permitted.