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Fully Qualified Domain Name
 (
FQDN
) of your NTP 
server instead of Ubuntu's servers.
Before we close out this chapter, there is one additional change you should consider 
implementing to the 
/etc/ntp.conf
 file. Look for the following line within that file:
#restrict 192.168.123.0 mask 255.255.255.0 notrust 
Change this line by first uncommenting it, changing the network address and subnet 
mask to match the details for your network, and then remove the 
notrust
 keyword 
at the end. The line should look similar to the following, depending on your network 
configuration:

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restrict 192.168.1.0 mask 255.255.255.0 
So, what does this option do for us? Basically, we're limiting access to our NTP 
server to local clients only, and we're only allowing read-only access for security 
purposes.
Now that you have a working NTP server, feel free to experiment and point your 
existing nodes to it and have them synchronize. To have devices on your network 
start using your new NTP server, it's as simple as configuring your DHCP server 
to point your devices to the IP address or DNS name of your NTP server as the 
designated time server for your network, and they'll start synchronizing with it as 
soon as their DHCP leases expire and renew. If you're utilizing the 
isc-dhcp-server
 
service we set up earlier for DHCP in your network, we can add the relevant option 
(shown in bold) to our 
/etc/dhcp/dhcpd.conf
 file:
subnet 192.168.1.0 netmask 255.255.255.0 {
  range 192.168.1.100 192.168.1.240;
  option routers 192.168.1.1;
  option domain-name-servers 192.168.1.1;