Workshop

for a TCP/IP address in a large network was not practical, so TCP/IP designers invented a method of
translating a MAC address into a larger, more unique value. This obviously isn’t necessary with today’s
MAC addresses, but this translation hangs on to this day.
Duplicate TCP/IP addresses can make your TCP/IP-dependent programs stop functioning. If you assign
your own TCP/IP addresses, be sure to keep them unique.
DHCP (Dynamic Host Configuration Protocol) server programs can dynamically hand out IP numbers to
computers. This can be a blessing or a curse—would you like it if your phone number changed every few
days? On the other hand, DHCP automates the addressing process and tends to eliminate addressduplication
errors. You can find practical troubleshooting information on DHCP in Hour 11, “Windows
Networking Basics.”
Like a phone number, TCP/IP has its network number (or “area code”) built right into it. Unlike a phone
number, though, the “area code” is the longer part of the number. The shorter part of the number
(referred to as the node or host number) is the local phone number, with the first part of the address
referring to the “area code,” known as the network number. Because TCP/IP network numbers don’t
have to be a specific length, the length of the network number is calculated with a network mask, a
number that mathematically specifies how long the network number is.
Fortunately, most small TCP/IP networks have the same network mask, so you can calculate your
network numbers just by following an example rather than engaging in horrible binary arithmetic.
The most common use of a network mask in troubleshooting is during configuration verification. Because
an incorrect network mask can cause a workstation to malfunction, making sure that a problem PC’s
network mask is the same as the others is important.
For a network mask whose numbers are all 255, it’s pretty easy. Simply write down the IP number with
the network mask beneath it; each number that matches up to a 255 is a network number. The remaining
number is the node number. (See Figure 1.3 for an example of this.)