ref https://www.hiroom2.com/2016/08/12/ubuntu-16-04-install-package-from-dvd/

$ sudo mount -t iso9660 /dev/cdrom /media/cdrom
mount: /dev/sr0 is write-protected, mounting read-only

Add cdrom to repository list with apt-cdrom.

$ sudo apt-cdrom -m -d /media/cdrom add
Using CD-ROM mount point /media/cdrom/
Identifying... [1a235e539972579c22a0bec9a077f7c3-2]
Scanning disc for index files...
Found 4 package indexes, 0 source indexes, 0 translation indexes and 1 signatures
This disc is called:
'Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)'
Copying package lists...gpgv: Signature made Thu 21 Apr 2016 07:29:18 AM JST using DSA key ID FBB75451
gpgv: Good signature from "Ubuntu CD Image Automatic Signing Key <cdimage@ubuntu.com>"
Reading Package Indexes... Done
Writing new source list
Source list entries for this disc are:
deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted
Repeat this process for the rest of the CDs in your set.    

The apt-cdrom changes /etc/apt/sources.list. For removing cdrom from repository list, remove "deb cdrom:" statement in /etc/apt/sources.list.

$ diff -uprN /etc/apt/sources.list{.org,}
--- /etc/apt/sources.list.org   2016-08-12 10:50:26.281895047 +0900
+++ /etc/apt/sources.list       2016-08-12 10:50:42.603934453 +0900
@@ -2,6 +2,7 @@

 # See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
 # newer versions of the distribution.
+deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted
 deb http://jp.archive.ubuntu.com/ubuntu/ xenial main restricted
 deb-src http://jp.archive.ubuntu.com/ubuntu/ xenial main restricted

@@ -51,3 +52,4 @@ deb http://security.ubuntu.com/ubuntu xe
 deb-src http://security.ubuntu.com/ubuntu xenial-security universe
 deb http://security.ubuntu.com/ubuntu xenial-security multiverse
 deb-src http://security.ubuntu.com/ubuntu xenial-security multiverse