Linux with Operating System Concepts

*
/
dhcpd.
conf.sample
(where 
*
denotes the version, for instance 
dhcp-4.1.1
). As with most 
conf files, you will have to modify the directives to fit your particular situation. These 
directives are listed in Table 12.4.
The conf file begins with global directives that apply to all subnets of the local area net-
work. The 
domain-name
and 
domain-name-servers
directives might be defined 
here if they cover the entire network. The conf file will then contain one or more subnet 
entries. Each subnet entry includes a netmask and the range of IP addresses that the DHCP 

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Linux with Operating System Concepts
will be able to issue to the clients on that subnet. Options include the broadcast IP address 
and router IP address.
Your DHCP may have multiple sets of entries if the DHCP is responsible for multi-
ple subnetworks. What follows is an example where the DHCP server hosts two subnets, 
10.11.0.0 and 10.11.128.0.
option domain-name somecompany.com;
option domain-name-servers 10.11.1.1 10.11.1.2;
subnet 10.11.0.0 netmask 255.255.128.0 {
range 10.11.12.0 10.11.12.20;
option broadcast-address 10.11.12.22;
option routers 10.11.12.21;
}
subnet 10.11.128.0 netmask 255.255.128.0 {
range 10.11.128.129 10.11.128.253;
broadcast-address 10.11.128.255;
option routers 10.11.128.254;
}
The two entries shown above specify two subnets that the DHCP server handles. You 
will notice that both subnets use the same netmask (255.255.128.0 corresponds to the first 
17 bits of the 32-bit address, that is, the network address is made up of the first 17 bits, 
and the machine address is the remaining 15 bits). This DHCP is allowed to hand out 
IP addresses within the range 10.11.12.0–10.11.12.20 for the first subnet and addresses of 
10.11.128.129–10.11.128.253 for the second subnet. The two subnets each have their own 
broadcast device and their own router; so, there are two sets of addresses, one per entry. 
TABLE 12.4 
DHCP Directives