The following is the second installment of a multi-part series on the fundamentals of routing. Each tip is excerpted from Routing First-Step by William Parkhurst, published by Cisco Press. Check back frequently for the next installment, or go to the main series page for all the installments.
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About the book
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Routing First-Step explains the basics of Internet routing in language all of us can understand. This book takes you on a guided tour of routing, starting with systems you are familiar with: the postal system, the telephone system, and the interstate highway system. From there, you'll learn routing simply and easily. Whether you are looking to take your first step into a career in networking or are interested only in gaining knowledge of the technology, this book is for you!
Author William R. Parkhurst, Ph.D., CCIE, manages the CCIE Development group at Cisco Systems. The CCIE Development group is responsible for all new CCIE written qualification and laboratory exams. Prior to joining the CCIE team, Bill was a Consulting Systems Engineer supporting the Sprint Operation. He first became associated with Cisco Systems while a Professor of Electrical and Computer Engineering at Wichita State University. In conjunction with Cisco Systems, WSU established the first CCIE Preparation Laboratory.
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Internet addressing protocol
The Internet addressing protocol, or Internet Protocol (IP), is an additional
addressing scheme. By following this protocol you can have an address with a
structure that will allow you to build a scalable data communication system analogous
to the postal system. Because this discussion is about computers, you need to
decide how many bits you need for an Internet address, and how many Internet
addresses you need. Computers can easily work with byte size pieces of data, so
the number of bits in the IP should be a multiple of 8 bits or 1 byte. How many IP
addresses will you need? That is, and was, a difficult question. When the IP was
developed more than 20 years ago, the PC was not common, and it was difficult to
imagine the explosion that would take place in the number of computers used
throughout the world.
If you placed yourself back in 1980, and had to determine the size, in bits, of an IP
address, what would you have picked? You might have started by determining
how many addresses are possible based on the number of bytes that are used. And
you might have created a table similar to Table 3-1.
Table 3-1 - Number of IP Addresses Versus Number of Bytes
In 1980, there were more than 256 computers in use, so 1 byte would not be sufficient. Two bytes would give us 65,280 more addresses, but the number is still not
sufficient. Although 3 bytes allow more than 16 million addresses, you know that
computers are happier with even numbers of things than odd numbers of things.
An ideal size is 4 bytes. Four is an even number and you can have in excess of
4 billion IP addresses, which should be sufficient. Now that you have settled on
using a 32-bit address for the Internet address, you next need to determine a structure
for those 32 bits.
The postal addressing and delivery schemes worked quite well for mail delivery,
so let's try and impose the same type of structure on the Internet addresses. You
know that a postal address has two components. The first component consisted of
the state, city, and street names. The second component was the street or house
number. Although the entire address was needed to identify a particular endpoint,
or house, you did not need the street number for delivery until the mail reached
the street containing the house. Using the same philosophy for the Internet
address, use part of the 32 bits to designate the LAN or local network where the
computer resides (the network address); and the remaining bits in the address to
identify a particular computer, or host (the host address), on that LAN.
The next step is to determine how many bits to use for the network address and
how many bits to use to identify a computer, or host, on that network. The easiest
approach is to work with bytes, and then use the dotted decimal notation to represent
network and host addresses using decimal numbers. You can't use all the
bytes for the network address, and you can't use all the bytes for the computer
address so the possibilities that remain are listed in Table 3-2.
Table 3-2 - Internet Address Structures
After some thought, you decide that you want to make some modifications to the
range of host addresses. It would be nice if you had a broadcast capability where a
message could be sent to every host on a network. Therefore, you need a broadcast
address for the network, and you want the broadcast address to be easy to
remember. This can be achieved by using a host address of all 1s for the broadcast
address. In addition, you want an address that points to the network itself, or
"this" network. This can be achieved by using a host address of all 0s. To accommodate
the new broadcast and "this" network addresses, the number of hosts
listed in Table 3-2 must now be reduced by two. (See Table 3-3.)
Table 3-3 - Number of Hosts Possible
Try working through the next exercises to reinforce your understanding.
For the first type or class of networks that use 1 byte for the network address and
3 bytes for the host address, the range of addresses in dotted decimal notation are:
0.0.0.1–0.255.255.254 for network 0
1.0.0.1–1.255.255.254 for network 1
2.0.0.1–2.255.255.254 for network 2
...
255.0.0.1–255.255.255.254 for network 255
1. What is the address for host 8 on network 129?
Answer: 129.0.0.8
2. What is the broadcast address for network 129?
Answer: 129.255.255.255
For the second class of networks that use 2 bytes for both the network and host
addresses, the range of addresses in dotted decimal notation are
0.0.0.1–0.0.255.254 for network 0
0.1.0.1–0.1.255.254 for network 1
...
255.254.0.1–255.254.255.254 for network 65,534
255.255.0.1–255.255.255.254 for network 65,535
3. What is the address for host 8 on the 258th network?
Answer: Answer.1.1.0.8. The 258th network is network number 257
because network numbering started at 0; 257 in binary = 0000 0001 0000
0001 = 256 + 1 = 257 = 1.1 in dotted decimal notation.
4. What is the broadcast address for the 258th network?
Answer: 1.1.255.255
For the third class of networks that use 1 byte for the host address and
3 bytes for the network address, the range of addresses in dotted decimal
notation are
0.0.0.1–0.0.0.254 for network 0
0.0.1.1–0.0.1.254 for network 1
...
0.1.0.1–0.1.0.254 for network 256
...
1.0.0.1–1.0.0.254 for network 65536
...
255.255.255.1–255.255.255.254 for network 16,777,215
5. What is the address for host 8 on network 25?
Answer: 0.0.25.8
6. What is the address for host 12 on network 103,481?
Answer: 103,481 in hexadecimal = 019439.0116 = 110.94
16 = 14810.3916 = 5710.103,481 in dotted decimal = 1.148.57 so the address for host 12 on
network 103,481 is 1.148.57.12
7. What is the broadcast address for network 103,481?
Answer: 1.148.57.255
Classful IP Addresses
You must decide which scheme you are going to use for the Internet addresses.
You could pick just one, but why not use all three? That way, you would have the
flexibility of having networks with a few hosts (254), networks with a moderate
number of hosts (65534), and networks with many hosts (16,777,214). This does
sound like a good idea, but you need to be able to mix the three address types. A
simple way is to use part of the first byte to signal the type of address used. The
first byte, like all bytes, contains 8 bits. Using the first few bits to identify the type
of network gives you the following rules:
If the first, or most significant, bit of the first byte is 0, then 1 byte is used for
the network address and 3 bytes are used for the host, broadcast, and "this"
network address.
If the most significant bit of the first byte is 1 and the next bit is 0, 2 bytes
are used for the network address and 2 bytes are used for the host, broadcast,
and "this" network address.
If the first 2 bits of the first byte are 1, then 3 bytes are used for the network
address and 1 byte is used for the host, broadcast, and "this" network
address.
Three types of addresses are called Class A, B, and C. In a Class A address, the
most significant bit of the first byte is 0. Additionally, you want to reserve two
addresses from the Class A address space. Address 0.0.0.X (where X can be any
value from 0–255) is reserved. Address 127.X.X.X is reserved for what is called a
loopback address. The loopback address is used by a host to send a message to
itself without even being connected to a network. This can be used for testing
applications without interfering with the network. So the range of values for the
first byte in a Class A address is 0000–0001–0111 1110 or 1–126 (0 and 127 are
reserved).
The entire range of Class A addresses is – 126.255.255.255.
In a Class B address, the most significant bit of the first byte is 1 and the next bit is
0. So the range of values for the first byte in a Class B address is 1000 0000–1011
1111 or 128–191.
The entire range of Class B addresses is: 128.0.0.0–191.255.255.255.
In a Class C address, the first two most significant bits of the first byte are 1 and
the next bit is 0. So the range of values for the first byte in a Class C address is
1100 0000–1101 1111 or 192–223.
The entire range of Class C addresses is: 192.0.0.0–223.255.255.255.
This Internet Protocol addressing scheme is called classful
because every address falls into one of three classes of addresses as summarized in Table 3-4.
Table 3-4 - Classful Internet Protocol Address Ranges
A Class A address is identified by the first bit being a 0, as follows:
0XXX XXXX
A Class B address is identified by the first 2 bits being 1 0, as follows:
10XX XXXX
And a Class C address has the first 3 bits set to 1 1 0, as follows:
110X XXXX
One question might have popped into your mind at this point: What about
addresses that are not Class A, B, or C? In other words, what about addresses
where the first four bits are 1 1 1 0 or 1 1 1 1? Addresses beginning with 1110 are
a different class of addresses, Class D, which you will learn about in Chapter 9,
"Multicast—What the Post Office Can't Do." The Class D, or multicast, address
space is in the range 1110 0000–1110 1111 or 224–239.
Addresses beginning with 1111 are reserved for future use and cover the remaining
address space starting at 240.0.0.0.
Figure 3-4 summarizes the structure of the classful IP addresses scheme.
Figure 3-4 - Classful IP Addressing Structures
Private IP Addresses
The final addition to the Internet addressing protocol is that of private IP
addresses. A public IP address is one that is reachable on the Internet, and therefore
must be globally unique (two computers cannot use the same public IP
address). You can have LANs that are not connected to the Internet, but the computers
on these LANs are using IP for communication. It doesn't make sense to
waste public IP addresses on these computers, so a range of addresses has been set
aside for these private networks to use. (See Table 3-5.) Because they are private,
the same addresses can be used on more than one LAN with the realization that
communication between LANs using the same private IP addresses is not possible.
Table 3-5 - Private IP Address Ranges
All parts reproduced from the book Routing First-Step, ISBN 1587201224, Copyright 2005, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses. Visit www.ciscopress.com for a detailed description and to learn how to purchase this title.
