I have two machines of Ubuntu 16.04.3 systems A and B.
First, I execute
apt update && apt-get install XXX YYY ZZZ on the machine A and I can get all deb packages at
/var/cache/apt/archives/. Then I copy all of these deb packages from A to B and on the machine B, I execute
apt update && apt install ./debsFromA/*.deb to install these deb packages locally.
However, I found that the machine B will redownload all of these deb packages from the ubuntu official siteweb (http://us.archive.ubuntu.com/ubuntu/ etc).
So I have offered all of deb packages, why would it still download them again?