When you configure the TCP/IP protocol on a Microsoft
Windows computer, an IP address, subnet mask, and usually a default
gateway are required in the TCP/IP configuration settings.
configure TCP/IP correctly, it is necessary to understand how TCP/IP
networks are addressed and divided into networks and subnetworks. This
article is intended as a general introduction to the concepts of IP
networks and subnetting. A glossary is included at the end of article.
The success of TCP/IP as the network protocol of the
Internet is largely because of its ability to connect together networks of
different sizes and systems of different types. These networks are
arbitrarily defined into three main classes (along with a few others) that
have predefined sizes, each of which can be divided into smaller
subnetworks by system administrators. A subnet mask is used to divide an
IP address into two parts. One part identifies the host (computer), the
other part identifies the network to which it belongs. To better
understand how IP addresses and subnet masks work, look at an IP (Internet
Protocol) address and see how it is organized.
IP addresses: Networks and hosts
An IP address is a 32-bit number
that uniquely identifies a host (computer or other device, such as a
printer or router) on a TCP/IP network.
IP addresses are normally
expressed in dotted-decimal format, with four numbers separated by
periods, such as 192.168.123.132. To understand how subnet masks are used
to distinguish between hosts, networks, and subnetworks, examine an IP
address in binary notation.
For example, the dotted-decimal IP
address 192.168.123.132 is (in binary notation) the 32 bit number
110000000101000111101110000100. This number may be hard to make sense of,
so divide it into four parts of eight binary digits.
bit sections are known as octets. The example IP address, then, becomes
11000000.10101000.01111011.10000100. This number only makes a little more
sense, so for most uses, convert the binary address into dotted-decimal
format (192.168.123.132). The decimal numbers separated by periods are the
octets converted from binary to decimal notation.
For a TCP/IP wide
area network (WAN) to work efficiently as a collection of networks, the
routers that pass packets of data between networks do not know the exact
location of a host for which a packet of information is destined. Routers
only know what network the host is a member of and use information stored
in their route table to determine how to get the packet to the destination
host's network. After the packet is delivered to the destination's
network, the packet is delivered to the appropriate host.
process to work, an IP address has two parts. The first part of an IP
address is used as a network address, the last part as a host address. If
you take the example 192.168.123.132 and divide it into these two parts
you get the following:
192.168.123.0 - network address.
0.0.0.132 - host address.
The second item, which is required for TCP/IP to work,
is the subnet mask. The subnet mask is used by the TCP/IP protocol to
determine whether a host is on the local subnet or on a remote
In TCP/IP, the parts of the IP address that are used as
the network and host addresses are not fixed, so the network and host
addresses above cannot be determined unless you have more information.
This information is supplied in another 32-bit number called a subnet
mask. In this example, the subnet mask is 255.255.255.0. It is not obvious
what this number means unless you know that 255 in binary notation equals
11111111; so, the subnet mask is:
Lining up the IP address and the subnet mask together, the
network and host portions of the address can be separated:
11000000.10101000.01111011.10000100 -- IP address (192.168.123.132)
11111111.11111111.11111111.00000000 -- Subnet mask (255.255.255.0)
The first 24 bits (the number of ones in the subnet mask) are
identified as the network address, with the last 8 bits (the number of
remaining zeros in the subnet mask) identified as the host address. This
gives you the following:
11000000.10101000.01111011.00000000 -- Network address (192.168.123.0)
00000000.00000000.00000000.10000100 -- Host address (000.000.000.132)
So now you know, for this example using a 255.255.255.0 subnet
mask, that the network ID is 192.168.123.0, and the host address is
0.0.0.132. When a packet arrives on the 192.168.123.0 subnet (from the
local subnet or a remote network), and it has a destination address of
192.168.123.132, your computer will receive it from the network and
Almost all decimal subnet masks convert to binary
numbers that are all ones on the left and all zeros on the right. Some
other common subnet masks are:
Internet RFC 1878 (available from http://www.internic.net/(http://www.internic.net)
) describes the valid subnets
and subnet masks that can be used on TCP/IP networks.
Internet addresses are allocated by the InterNIC
), the organization that
administers the Internet. These IP addresses are divided into classes. The
most common of these are classes A, B, and C. Classes D and E exist, but
are not generally used by end users. Each of the address classes has a
different default subnet mask. You can identify the class of an IP address
by looking at its first octet. Following are the ranges of Class A, B, and
C Internet addresses, each with an example address:
||Class A networks use a default subnet mask of
255.0.0.0 and have 0-127 as their first octet. The address
10.52.36.11 is a class A address. Its first octet is 10, which is
between 1 and 126, inclusive.|
||Class B networks use a default subnet mask of
255.255.0.0 and have 128-191 as their first octet. The address
172.16.52.63 is a class B address. Its first octet is 172, which is
between 128 and 191, inclusive.|
||Class C networks use a default subnet mask of
255.255.255.0 and have 192-223 as their first octet. The address
192.168.123.132 is a class C address. Its first octet is 192, which
is between 192 and 223, inclusive.|
scenarios, the default subnet mask values do not fit the needs of the
organization, because of the physical topology of the network, or because
the numbers of networks (or hosts) do not fit within the default subnet
mask restrictions. The next section explains how networks can be divided
using subnet masks.
A Class A, B, or C TCP/IP network can be further
divided, or subnetted, by a system administrator. This becomes necessary
as you reconcile the logical address scheme of the Internet (the abstract
world of IP addresses and subnets) with the physical networks in use by
the real world.
A system administrator who is allocated a block of
IP addresses may be administering networks that are not organized in a way
that easily fits these addresses. For example, you have a wide area
network with 150 hosts on three networks (in different cities) that are
connected by a TCP/IP router. Each of these three networks has 50 hosts.
You are allocated the class C network 192.168.123.0. (For illustration,
this address is actually from a range that is not allocated on the
Internet.) This means that you can use the addresses 192.168.123.1 to
192.168.123.254 for your 150 hosts.
Two addresses that cannot be
used in your example are 192.168.123.0 and 192.168.123.255 because binary
addresses with a host portion of all ones and all zeros are invalid. The
zero address is invalid because it is used to specify a network without
specifying a host. The 255 address (in binary notation, a host address of
all ones) is used to broadcast a message to every host on a network. Just
remember that the first and last address in any network or subnet cannot
be assigned to any individual host.
You should now be able to give
IP addresses to 254 hosts. This works fine if all 150 computers are on a
single network. However, your 150 computers are on three separate physical
networks. Instead of requesting more address blocks for each network, you
divide your network into subnets that enable you to use one block of
addresses on multiple physical networks.
In this case, you divide
your network into four subnets by using a subnet mask that makes the
network address larger and the possible range of host addresses smaller.
In other words, you are 'borrowing' some of the bits usually used for the
host address, and using them for the network portion of the address. The
subnet mask 255.255.255.192 gives you four networks of 62 hosts each. This
works because in binary notation, 255.255.255.192 is the same as
1111111.11111111.1111111.11000000. The first two digits of the last octet
become network addresses, so you get the additional networks 00000000 (0),
01000000 (64), 10000000 (128) and 11000000 (192). (Some administrators
will only use two of the subnetworks using 255.255.255.192 as a subnet
mask. For more information on this topic, see RFC 1878.) In these four
networks, the last 6 binary digits can be used for host
Using a subnet mask of 255.255.255.192, your
192.168.123.0 network then becomes the four networks 192.168.123.0,
192.168.123.64, 192.168.123.128 and 192.168.123.192. These four networks
would have as valid host addresses:
Remember, again, that binary host addresses with all ones or all
zeros are invalid, so you cannot use addresses with the last octet of 0,
63, 64, 127, 128, 191, 192, or 255.
You can see how this works by
looking at two host addresses, 192.168.123.71 and 192.168.123.133. If you
used the default Class C subnet mask of 255.255.255.0, both addresses are
on the 192.168.123.0 network. However, if you use the subnet mask of
255.255.255.192, they are on different networks; 192.168.123.71 is on the
192.168.123.64 network, 192.168.123.133 is on the 192.168.123.128 network.
If a TCP/IP computer needs to communicate with a
host on another network, it will usually communicate through a device
called a router. In TCP/IP terms, a router that is specified on a host,
which links the host's subnet to other networks, is called a default
gateway. This section explains how TCP/IP determines whether or not to
send packets to its default gateway to reach another computer or device on
When a host attempts to communicate with another
device using TCP/IP, it performs a comparison process using the defined
subnet mask and the destination IP address versus the subnet mask and its
own IP address. The result of this comparison tells the computer whether
the destination is a local host or a remote host.
If the result of
this process determines the destination to be a local host, then the
computer will simply send the packet on the local subnet. If the result of
the comparison determines the destination to be a remote host, then the
computer will forward the packet to the default gateway defined in its
TCP/IP properties. It is then the responsibility of the router to forward
the packet to the correct subnet.
TCP/IP network problems are often caused by
incorrect configuration of the three main entries in a computer's TCP/IP
properties. By understanding how errors in TCP/IP configuration affect
network operations, you can solve many common TCP/IP
Incorrect Subnet Mask: If a network uses a subnet mask
other than the default mask for its address class, and a client is still
configured with the default subnet mask for the address class,
communication will fail to some nearby networks but not to distant ones.
As an example, if you create four subnets (such as in the subnetting
example) but use the incorrect subnet mask of 255.255.255.0 in your TCP/IP
configuration, hosts will not be able to determine that some computers are
on different subnets than their own. When this happens, packets destined
for hosts on different physical networks that are part of the same Class C
address will not be sent to a default gateway for delivery. A common
symptom of this is when a computer can communicate with hosts that are on
its local network and can talk to all remote networks except those that
are nearby and have the same class A, B, or C address. To fix this
problem, just enter the correct subnet mask in the TCP/IP configuration
for that host.
Incorrect IP Address: If you put computers with IP
addresses that should be on separate subnets on a local network with each
other, they will not be able to communicate. They will try to send packets
to each other through a router that will not be able to forward them
correctly. A symptom of this problem is a computer that can talk to hosts
on remote networks, but cannot communicate with some or all computers on
their local network. To correct this problem, make sure all computers on
the same physical network have IP addresses on the same IP subnet. If you
run out of IP addresses on a single network segment, there are solutions
that go beyond the scope of this article.
Gateway: A computer configured with an incorrect default gateway will be
able to communicate with hosts on its own network segment, but will fail
to communicate with hosts on some or all remote networks. If a single
physical network has more than one router, and the wrong router is
configured as a default gateway, a host will be able to communicate with
some remote networks, but not others. This problem is common if an
organization has a router to an internal TCP/IP network and another router
connected to the Internet.
Two popular references on TCP/IP are:
Illustrated, Volume 1: The Protocols," Richard Stevens, Addison Wesley,
"Internetworking with TCP/IP, Volume 1: Principles, Protocols,
and Architecture," Douglas E. Comer, Prentice Hall, 1995
strongly recommended that a system administrator responsible for TCP/IP
networks have at least one of these references available.
Broadcast address -- An IP address with a host portion
that is all ones.
Host -- A computer or other device on a TCP/IP
Internet -- The global collection of networks that are
connected together and share a common range of IP
InterNIC -- The organization responsible for
administration of IP addresses on the Internet.
IP -- The network
protocol used for sending network packets over a TCP/IP network or the
IP Address -- A unique 32-bit address for a host on a
TCP/IP network or internetwork.
Network -- There are two uses of
the term network in this article. One is a group of computers on a single
physical network segment; the other is an IP network address range that is
allocated by a system administrator.
Network address -- An IP
address with a host portion that is all zeros.
Octet -- An 8-bit
number, 4 of which comprise a 32-bit IP address. They have a range of
00000000-11111111 that correspond to the decimal values 0-
Packet -- A unit of data passed over a TCP/IP network or wide
RFC (Request for Comment) -- A document used to
define standards on the Internet.
Router -- A device that passes
network traffic between different IP networks.
Subnet Mask -- A
32-bit number used to distinguish the network and host portions of an IP
Subnet or Subnetwork -- A smaller network created by
dividing a larger network into equal parts.
TCP/IP -- Used broadly,
the set of protocols, standards and utilities commonly used on the
Internet and large networks.
Wide area network (WAN) -- A large
network that is a collection of smaller networks separated by routers. The
Internet is an example of a very large WAN.