Apple Mac OS X Server (Administrator’s Guide) - Manual (Page 476)

476 Chapter 11
Creating Subnets
Subnets are groupings of computers on the same network that simplify administration. You
can organize subnets any way that is useful to you. For example, you can create subnets for
different groups within your organization or for different floors of a building. Once you have
grouped client computers into subnets, you can configure options for all the computers in a
subnet at one time instead of setting options for individual client computers. Each subnet
needs a way to connect to the other subnets. A hardware device called a router typically
connects subnets.
Assigning IP Addresses Dynamically
With dynamic allocation, an IP address is assigned for a limited period of time (the lease
period) or until the client computer doesn’t need the IP address, whichever comes first. By
using short leases, DHCP can reassign IP addresses on networks that have more computers
than available IP addresses.
Using Static IP Addresses
Static IP addresses are assigned to a computer or device once and then do not change. You
may want to assign static IP addresses to computers that must have a continuous Internet
presence, such as Web servers. Other devices that need to be continuously available to
network users, such as printers, may also benefit from static IP addresses.
Static IP addresses can be set up either by manually entering the IP address on the computer
or device or by configuring DHCP to provide the same address to a specific computer or
device on each request. DHCP-assigned addresses allow configuration changes at the DHCP
server. Manually configured static IP addresses avoid possible issues certain services may have
with DHCP-assigned addresses and avoid the delay required for DHCP to process the request.
Server Settings does not provide a way to assign static IP addresses using the BootP protocol
(the protocol underlying DHCP). To assign static IP addresses, you can use the NetInfo
Manager application in Mac OS X to create the appropriate properties in the local NetInfo
database. See “Configuring Static Ports for Shared NetInfo Domains” on page 113 for more
information on setting up static IP addresses on local networks.
Locating the DHCP Server
When a client computer looks for a DHCP server, it broadcasts a message. If your DHCP
server is on a different subnet from the client computer, you must make sure the routers that
connect your subnets can forward the client broadcasts and the DHCP server responses. If
you have a relay agent or a router on your network that can relay BootP communications, it
will work for DHCP. If you don’t have a relay, you need to place the DHCP server on the
same subnet as your clients.