Q164015: Understanding TCP/IP Addressing and Subnetting Basics
Article: Q164015
Product(s): Windows for Workgroups and Windows NT Networking Issues
Version(s): 2000,3.0,3.1,3.5,3.51,4.0
Operating System(s):
Keyword(s): kbnetwork kbusage win95 win98
Last Modified: 28-MAR-2002
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The information in this article applies to:
- Microsoft Windows NT Server versions 3.1, 3.5, 3.51, 4.0
- Microsoft Windows NT Workstation versions 3.1, 3.5, 3.51, 4.0
- Microsoft Windows NT Advanced Server
- Microsoft TCP/IP-32 for Windows for Workgroups
- Microsoft Windows 95
- Microsoft Windows 98
- Microsoft Network Client for MS-DOS version 3.0
- Microsoft Windows 2000 Advanced Server
- Microsoft Windows 2000 Professional
- Microsoft Windows 2000 Server
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SUMMARY
=======
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.
To 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.
MORE INFORMATION
================
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.
These eight 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.
For this 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. Network
.132 Host
-or-
192.168.123.0 - network address.
0.0.0.132 - host address.
Subnet Mask
-----------
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 network.
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:
11111111.11111111.11111111.0000000
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 process it.
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:
Decimal Binary
255.255.255.192 1111111.11111111.1111111.11000000
255.255.255.224 1111111.11111111.1111111.11100000
Internet RFC 1878 (available from http://ds.internic.net) describes the valid
subnets and subnet masks that can be used on TCP/IP networks.
Network Classes
---------------
Internet addresses are allocated by the InterNIC (http://www.internic.net), 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.
In some 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.
Subnetting
----------
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 addresses.
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:
192.168.123.1-62
192.168.123.65-126
192.168.123.129-190
192.168.123.193-254
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.
Default Gateways
----------------
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 the network.
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.
Troubleshooting
---------------
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
problems.
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.
Incorrect Default 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.
REFERENCES
==========
Two popular references on TCP/IP are:
"TCP/IP Illustrated, Volume 1: The Protocols," Richard Stevens, Addison Wesley,
1994
"Internetworking with TCP/IP, Volume 1: Principles, Protocols, and Architecture,"
Douglas E. Comer, Prentice Hall, 1995
It is strongly recommended that a system administrator responsible for TCP/IP
networks have at least one of these references available.
Glossary
--------
Broadcast address -- An IP address with a host portion that is all ones.
Host -- A computer or other device on a TCP/IP network.
Internet -- The global collection of networks that are connected together and
share a common range of IP addresses.
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 Internet.
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- 255.
Packet -- A unit of data passed over a TCP/IP network or wide area network.
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 address.
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.
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