enough to have its own HR department and payroll section, you could put
those departments’ hosts on their own subnet and use your router
configuration to limit the hosts that can connect to this subnet. This
configuration prevents networked workers who are not members of the
designated departments from being able to view some of the confidential
information the HR and payroll personnel work with.
Subnet use also enables your network to grow beyond 254 hosts and share IP
addresses. With proper routing configuration, users might not even know they
are on a different subnet from their co-workers. Another common use for
subnetting is with networks that cover a wide geographic area. It is not
practical for a company with offices in Chicago and London to have both
offices on the same subnet, so using a separate subnet for each office is the
best solution.
Subnet Masks
TCP/IP uses subnet masks to show which part of an IP address is the network
portion and which part is the host. Subnet masks are usually referred to as
netmasks. For a pure Class A network, the netmask is 255.0.0.0; for a Class B
network, the netmask is 255.255.0.0; and for a Class C network, the netmask
is 255.255.255.0. You can also use netmasks to deviate from the standard
classes.
By using customized netmasks, you can subnet your network to fit your
needs. For example, say that your network has a single Class C address. You
have a need to subnet your network. Although this is not possible with a
normal Class C subnet mask, you can change the mask to break your network
into subnets. By changing the last octet to a number greater than zero, you can
break the network into as many subnets as you need.
For more information on how to create customized subnet masks, see Day 6,
“The Art of Subnet Masking,” in Sams Teach Yourself TCP/IP Network
Administration in 21 Days. That chapter goes into great detail on how to
create custom netmasks and explains how to create an addressing cheat sheet
for hosts on each subnet. The Linux Network Administrators Guide, at
[http://www.tldp.org/LDP/nag2/index.html, also has good information about how to](http://www.tldp.org/LDP/nag2/index.html, also has good information about how to)
create subnets.
Broadcast, Unicast, and Multicast Addressing
Information can get to systems through three types of addresses: unicast,