This is a desktop system with a single wired NIC.  In the last week or so, when the system boots, cups comes up in a state that refuses HTTP requests from other hosts using the correct cups system hostname:

[root@ftgme2 ~]# systemctl status cups >/tmp/out
[root@ftgme2 ~]# cat /tmp/out
● cups.service - CUPS Scheduler
   Loaded: loaded (/usr/lib/systemd/system/cups.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2017-06-15 09:57:05 EDT; 50min ago
     Docs: man:cupsd(8)
 Main PID: 3270 (cupsd)
   Status: "Scheduler is running..."
   CGroup: /system.slice/cups.service
           ├─3270 /usr/sbin/cupsd -l
           └─3704 /usr/lib/cups/notifier/dbus dbus:// 

Jun 15 09:57:04 ftgme2 cupsd[3270]: Loading from cache...
Jun 15 09:57:04 ftgme2 cupsd[3270]: Loading from cache...
Jun 15 09:57:04 ftgme2 cupsd[3270]: Loading from cache...
Jun 15 09:57:04 ftgme2 cupsd[3270]: Loading from cache...
Jun 15 09:57:05 ftgme2 systemd[1]: Started CUPS Scheduler.
Jun 15 09:57:05 ftgme2 cupsd[3270]: REQUEST localhost - - "POST / HTTP/1.1" 200 349 Create-Printer-Subscriptions successful-ok
Jun 15 09:57:05 ftgme2 cupsd[3270]: REQUEST localhost - - "POST / HTTP/1.1" 200 176 Create-Printer-Subscriptions successful-ok
Jun 15 09:58:45 ftgme2 cupsd[3270]: REQUEST localhost - - "POST / HTTP/1.1" 200 413 Create-Printer-Subscriptions successful-ok
Jun 15 10:47:00 ftgme2 cupsd[3270]: [Client 17] Request from "192.168.3.105" using invalid Host: field "ftgme2:631".
Jun 15 10:47:00 ftgme2 cupsd[3270]: [Client 18] Request from "192.168.3.105" using invalid Host: field "ftgme2:631".

but

[root@ftgme2 ~]# hostname
ftgme2
[root@ftgme2 ~]# 

This looks like cups is coming up before the network has fully started.  The hostname ftgme2 is set via the DHCP exchange that assigns the IP address.

If you restart cups, all is well.

This is a fairly new error.  Marking as major since no newbie is going to know to restart cups.