This page quickly explain what is ftp.debian.org, and provides some resources.
If you are looking for a mirror, use the CDN instead
Use http://deb.debian.org/ to connect to a CDN that caches packages on a node closest to your geographic and network location.
What is ftp.xx.debian.org
ftp.xx.debian.org are the main mirrors used to distribute Debian (the packages repositories, but not necessarily the CD images).
https://www.debian.org/mirror/list - Official information related to Debian Mirrors
Which mirror to use?
Many people seem to think that ftp.debian.org is the canonical location of Debian packages and that it will be best for them to use that site for apt or for mirroring. This is *not true*.
ftp.debian.org is merely one of several servers that get updated from an internal Debian server. That address is presently located on a single server in the United States, and it still exists mainly for backwards compatibility.
In the future, it may get services reduced, or shut down, or converted into a globally load-balanced name, or whatever. Please don't use it.
If you're using it now, please switch to either the deb.debian.org CDN or a country-based DNS name such as ftp.us.debian.org, ftp.ca.debian.org, ftp.uk.debian.org, ...
don't use ftp.debian.org
Do not use ftp.debian.org, see:
An easy to parse list of mirrors