Information Technology Reference
In-Depth Information
Take, for instance, the address 172.31.0.0 with the default subnet mask 255.255.0.0. As
Table 8-2 showed, this subnet mask allows for a 16-bit host ID. With these 16 bits, you can
assign more than 65,000 host addresses, starting with 172.31.0.1 and ending with
172.31.255.254. (Remember that you can't assign an address with all 0 bits or all 1 bits in the
host ID, so you have to exclude 172.31.0.0 and 172.31.255.255 from the possible IP addresses
you can assign to a host.) The exact calculation for the number of hosts is 2
n
2;
n
is the number
of bits in the host ID. Being able to assign this many addresses might seem like an advantage if
you have a large network. However, having such a large address space assigned to a single net-
work has two distinct disadvantages: If you're actually using the number of computers the
address space affords (in this case, more than 65,000), communication efficiency suffers, and if
you aren't using the addresses, precious address space is wasted.
All computers and devices that share the same network ID in their IP address are said to be
in the same broadcast domain. A broadcast domain defines which devices must receive a packet
that's broadcast by any other device. A broadcast is a packet addressed to all computers on the
network. TCP/IP communication relies heavily on broadcast packets to perform a variety of
functions. For example, DHCP and ARP use broadcasts to perform their tasks. Every time a
computer receives a broadcast packet, the NIC generates an interrupt, causing the CPU to stop
what it's doing to read the packet. If the broadcast isn't relevant to the computer, the packet is
usually discarded. Now imagine 65,000 computers on the same broadcast domain; at any
moment, probably several thousand are sending broadcast packets. The amount of traffic gen-
erated and the additional CPU utilization would likely bring the network to a screeching halt.
Preventing this problem is where subnetting comes in.
If you do have 65,000 computers in your organization, instead of creating one large net-
work with the network address 172.31.0.0/16, you can divide this enormous network into
many smaller networks. For example, you can use 172.31.0.0/24, 172.31.1.0/24, and so forth
up to 172.31.255.0/24. This strategy makes 256 smaller networks with a maximum of 2
8
−
8
2,
or 254, devices per network. (With 24 network bits, there are 8 bits in the host ID because an
IP address has 32 bits total.) If a computer on one network needs to communicate with a com-
puter on another network, the packets are sent to a router that locates the network and for-
wards the data. Now the maximum size of your broadcast domain is only 254 computers,
which is more manageable.
Another reason to subnet is to conserve IP addresses. Companies that maintain Internet-
connected devices need public Internet addresses, which must be unique in the world—meaning
a public address can be assigned to only one device on the Internet. In the past, if a company had
four Web servers and two routers that needed public addresses, the only recourse an ISP had was
to assign a class C network address consisting of 254 possible host addresses, thereby wasting 248
addresses. By subnetting a network, the ISP can assign an address such as 198.60.123.0/29 that
uses only addresses 198.60.123.0 through 198.60.123.7, which satisfies the company's needs and
still makes addresses 198.60.123.8 through 198.60.123.254 available for other customers.
This section has by no means been a thorough discussion of subnetting, but it should pro-
vide the information you need to understand IP address configuration and the importance of
grouping computers by subnets when designing Active Directory sites.
−
For an online tutorial on subnetting, try
www.learntosubnet.com
.
Configuring Multiple IP Addresses
Windows OSs allow assigning multiple IP
addresses to a single network connection in the Advanced TCP/IP Settings dialog box shown in
Figure 8-7. As long as the address isn't assigned via DHCP, you can click the Add button and
enter a new IP address and subnet mask. Multiple IP addresses can be useful in these situations:
• The computer is hosting a service that must be accessed by using different addresses. For
example, a Web server can host multiple Web sites, each assigned a different IP address
and domain name.
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