[ntp:questions] broadcast ntpd server on Windows

Danny Mayer mayer at ntp.isc.org
Sat Jan 20 03:51:01 UTC 2007

wa6zvp wrote:
> mshetty at mail.com wrote:
>> Hi,
>> We have installed ntp-4.2.0a at mbg-fluxcap-v2-o-win32-setup.exe. We are
>> unable to use it as a broadcast server.
>> Would help if anyone who has has configured it as a broadcast server
>> can get back.
>> Every time we specify a broadcast address like in
>> ntp.conf, re-starting ntpd service results in an error like not Class D
>> address.

That's not an error. Class D addresses are multicast addresses and you
are not using a multicast address. is a broadcast
address. You must be running in debug mode unless that version emitted
that message anyway. It's really only meant for debugging.

>> We need a broadcast ntpd server on Windows. Incase there's any other
>> freeware available kindly let us know.
>> Thanks and Regards,
>> M Shetty
> Wouldn't your clients respond correctly if you used the multicast
> address that
> is assigned to NTP?  The normal address is, which I use.

There is no "normal" address. There are a range of reserved IPv4
addresses you can use any one of which is a valid multicast address.

> Not positive, but maybe if you just used the keyword 'broadcast',
> without an address,
> it might default to the standard broadcast address for the current
> network.

An address is *required* for the broadcast statement otherwise it would
not know which kind of 'casting to do and to what address. There is no
standard broadcast address.

Whether it is better to use broadcast or multicast depends a great deal
on the location of the recipient systems in the network. If every system
on the local LAN needs to receive packets then broadcasting to the local
LAN broadcast address is usually the best solution. If only a few
systems need it then multicast to a local multicast address with a low
hop count (ttl in IPv4 language) is the best solution or they can get it
directly by pointing the system's ntpd server to it.


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