netmask = ( aaa.bbb.ccc.ddd )
The netmask variable defines the partitioning of the subnet addresses on
your network. Its value is defined by your network administrator. On
most systems it is likely to be 255.255.255.0. This is used to
configure the network interface in netconfig.
See section actionsequence.
Every host on the internet has its own unique address. The addresses
are assigned hierarchically. Each network gets a domain name and
can attach something like 65,000 hosts to that network. Since this is
usually too many to handle in one go, every such network may be divided
up into subnets. The administrator of the network can decide how the
division into subnets is made. The decision is a trade-off between
having many subnets with few hosts, or many hosts on few subnets. This
choice is made by setting the value of a variable called netmask.
The netmask looks like an internet address. It takes the form:
aaa.bbb.ccc.mmm
The first two numbers `aaa.bbb' are the address of the domain. The
remainder `ccc.mmm' specifies both the subnet and the hostname.
The value of netmask tells all hosts on the network: how many of
the bits in the second half label different subnets and how many label
different hosts on each of the subnets?
The most common value for the netmask is `255.255.255.0'. It is most helpful to think of the netmask in terms of bits. Each base-10 number between 0-255 represents 8 bits which are either set or not set. Every bit which is set is a network address and every bit which is zero is part of a host address. The first two parts of the address `255.255' always takes these values. If the third number is `255', it means that the domain is divided up into 256 sub networks and then the remaining bits which are zero can be used to give 255 different host addresses on each of the subnets.
If the value had been `255.255.255.254', the network would be divided up into 2^15 subnets, since fifteen of the sixteen bits are one. The remaining bit leaves enough room for two addresses 0 and 1. One of those is reserved for broadcasts to all hosts, the other can be an actual host -- there would only be room for one host per subnet. This is a stupid example of course, the main point with the subnet mask is that it can be used to trade subnets for hosts per subnet. A value of `255.255.254.0' would allow 128 different subnets with 2*256-1 = 511 hosts on each.
We needn't be concerned with the details of the netmask here. Suffice it to say that its value is determined for your entire domain by the network administrator and each host has to be told what the value is.
Each host must also know what convention is used for the broadcast address. This is an address which hosts can send to if they wish to send a message to every other host on their subnet simultaneously. It is used a lot by services like NIS to ask if any hosts are willing to perform a particular service. There are two main conventions for the broadcast address: address zero (all host bits are zero) and the highest address on the subnet (all host bits are ones). The convention can be different on every subnet and it is decided by the network administrator. When you write a cfengine program you just specify the convention used on your subnet and cfengine works out the value of the broadcast address from the netmask and the host address See section broadcast. Cfengine works out the value of the broadcast address using the value of the netmask.
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