The basic syntax of an IPaddr resource is
This can be abbreviated simply to:
So, the following forms of addressing are legal:
address address/netmask address/netmask/broadcast address/netmask/interface address/netmask/interface/broadcast
By far the most common case is the simplest one:
In this form, appropriate defaults are taken as described below.
If the interface was omitted, we use the system routing tables to determine which interface would be used to route a packet to the given address. That is, IPaddr selects the preferred interface for sending packets to that subnet. If no such interface can be found, heartbeat will issue a message something like this:
ERROR: unable to find an interface for given-ip-address
If the CIDR netmask is omitted, we choose the netmask associated with the route to the subnet to which packets to the given IP address would be routed. That is, we simply look up the routing information for that address, and use it to compute the correct netmask (and other items as described below).
If the broadcast address was omitted, IPaddr defaults to the highest address in the subnet (the usual default).
188.8.131.52 184.108.40.206/24 (implies a 255.255.255.0 netmask) 220.127.116.11/24/18.104.22.168 22.214.171.124/24/eth0 126.96.36.199/24/eth0/188.8.131.52
The interface mentioned above does not include an alias number. Heartbeat assigns them dynamically according to the available alias names.
IPaddr has a limit of around 100 aliases it can manage for any given interface.
See http://www.doom.net/docs/netmask.html for a table explaining CIDR address format and their relationship to life, the universe and everything.