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15.17.5. Adding a Broadcast or Multicast Server Address

To add a broadcast or multicast address for sending, that is to say, the address to broadcast or multicast NTP packets to, make use of the broadcast command in the ntp.conf file.
The broadcast and multicast modes require authentication by default. See Section 15.6, “Authentication Options for NTP”.
The broadcast command takes the following form:
broadcast address
where address is an IP broadcast or multicast address to which packets are sent.
This command configures a system to act as an NTP broadcast server. The address used must be a broadcast or a multicast address. Broadcast address implies the IPv4 address By default, routers do not pass broadcast messages. The multicast address can be an IPv4 Class D address, or an IPv6 address. The IANA has assigned IPv4 multicast address and IPv6 address FF05::101 (site local) to NTP. Administratively scoped IPv4 multicast addresses can also be used, as described in RFC 2365 Administratively Scoped IP Multicast.