12.102.20 COULD HAVE THE SUBNET MASK 255.255.255.0 AND NETWORK ID...

184.12.102.20 could have the subnet mask 255.255.255.0 and network ID 184.12.102.0,

as opposed to the default subnet mask 255.255.0.0 with the network ID 184.12.0.0. This

allows an organization to subnet an existing class B network ID of 184.12.0.0 into

smaller subnets to match the actual configuration of their network.

Real World Classful Addressing and CIDR

Although classful IP addressing is important to understand, it is primarily interest-

ing only from a historical perspective. Most modern networks that use public

class A or B addresses are no longer organized by using the traditional classful

subnet mask. Originally, routers and routing protocols did not separately track

network IDs and subnet masks because memory for these devices was scarce and

expensive. Instead, classful routing was necessary because devices had to assume

the subnet mask based on the first octet. Today, memory is cheap, and every

router (and routing protocol) stores both network IDs and subnet masks in the

routing tables.

Private Addressing

Every network interface that is connected directly to the Internet must have an IP

address registered with the Internet Assigned Numbers Authority (IANA), which pre-

vents IP address conflicts between devices. If you are configuring a private network

that is not connected to the Internet or one that exists behind a firewall or proxy server,

you can configure devices on your network with private addresses and have only the

public address configured on the interface that is visible to the Internet.

Each address class has a range of private addresses available for general use:

■

Class A: 10.0.0.0 through 10.255.255.255

■

Class B: 172.16.0.0 through 172.31.255.255

■

Class C: 192.168.0.0 through 192.168.255.255

You can choose whichever range you like to use for your network and implement cus-

tom subnets as you see fit. None of these addresses is ever officially assigned to a pub-

licly accessible Internet host.

On the CD At this point, you should view three multimedia presentations: “Components of an IP Address,” “How IP Addresses are Wasted,” and “How Subnet Masks Work.” These pre-sentations are available in the Multimedia folder on the CD-ROM accompanying this book. Together, these presentations will strengthen your understanding of how IP addresses and subnet masks work.

How to Configure TCP/IP to Use a Static IP Address

By default, client computers running Windows 95 and later are configured to obtain

TCP/IP configuration information automatically. Automatic TCP/IP information is pro-

vided on a network using a DHCP server. When a client computer starts, it sends a

broadcast message to the network looking for a DHCP server that can provide IP