The representation of the IP addresses and port numbers in the PORT command and
PASV reply poses a challenge to FTP in traversing Network address translators (NAT).
The NAT device must alter these values, so that they contain the IP address of the
NAT ed client, and a port chosen by the NAT device for the data connection. The
new address and port will probably differ in length in their decimal representation
from the original address and port. Such translation is not usually performed in
most NAT devices, but special application layer gateways exist for this purpose.