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
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:
192.168.123.0 - 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 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:
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
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
Internet RFC 1878 (available from 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.
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.
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
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 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.
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.
Two popular references on TCP/IP are:
"TCP/IP Illustrated, Volume 1: The Protocols," Richard Stevens, Addison
"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.
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
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 area
RFC (Request for Comment) -- A document used to define standards on the
Router -- A device that passes network traffic between different IP
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