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This article is intended as a general introduction khổng lồ the concepts of Internet Protocol (IP) networks & subnetting. A glossary is included at the kết thúc of article.

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Applies to: Windows 10 - all editionsOriginal KB number: 164015


When you configure the TCP/IP.. protocol on a Windows computer, the TCP/IPhường. configuration settings require:

An IP addressA subnet maskA mặc định gateway

To configure TCP/IPhường correctly, it"s necessary to lớn understvà how TCP/IP. networks are addressed and divided into lớn networks and subnetworks.

The success of TCP/IP.. as the network protocol of the Internet is largely because of its ability lớn 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 sầu predefined sizes. Each of them can be divided inkhổng lồ 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 lớn which it belongs. To better underst& how IPhường addresses và subnet masks work, look at an IP address và see how it"s organized.

IPhường. addresses: Networks và hosts

An IPhường address is a 32-bit number. It uniquely identifies a host (computer or other device, such as a printer or router) on a TCP/IPhường network.

IPhường. addresses are normally expressed in dotted-decimal format, with four numbers separated by periods, such as To understvà how subnet masks are used to distinguish between hosts, networks, và subnetworks, examine an IP address in binary notation.

For example, the dotted-decimal IP.. address is (in binary notation) the 32-bit number 110000000101000111101110000100. This number may be hard to make sense of, so divide it inlớn four parts of eight binary digits.

These 8-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 inkhổng lồ dotted-decimal format ( The decimal numbers separated by periods are the octets converted from binary khổng lồ 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 don"t 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 và use information stored in their route table to lớn determine how lớn get the packet to lớn the destination host"s network. After the packet is delivered to the destination"s network, the packet is delivered khổng lồ the appropriate host.

For this process khổng lồ 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 and divide it inkhổng lồ these two parts, you get 192.168.123. Network .132 Host or - network address. - host address.

Subnet mask

The second thành tích, which is required for TCP/IP lớn work, is the subnet mask. The subnet mask is used by the TCP/IP. protocol lớn determine whether a host is on the local subnet or on a remote network.

In TCP/IP, the parts of the IPhường. address that are used as the network và host addresses aren"t fixed. Unless you have more information, the network & host addresses above can"t be determined. This information is supplied in another 32-bit number called a subnet mask. The subnet mask is in this example. It isn"t obvious what this number means unless you know 255 in binary notation equals 11111111. So, the subnet mask is 11111111.11111111.11111111.00000000.

Lining up the IPhường address and the subnet mask together, the network, and host portions of the address can be separated:

11000000.10101000.01111011.10000100 - IPhường address ( - Subnet mask (

The first 24 bits (the number of ones in the subnet mask) are identified as the network address. The last 8 bits (the number of remaining zeros in the subnet mask) are identified as the host address. It gives you the following addresses:

11000000.10101000.01111011.00000000 - Network address ( - Host address (

So now you know, for this example using a subnet mask, that the network ID is, & the host address is When a packet arrives on the subnet (from the local subnet or a remote network), and it has a destination address of, your computer will receive sầu it from the network và process it.

Almost all decimal subnet masks convert to binary numbers that are all ones on the left & all zeros on the right. Some other common subnet masks are:

Decimal Binary 1111111.11111111.1111111.11000000 1111111.11111111.1111111.11100000

Internet RFC 1878 (available from InterNIC-Public Information Regarding Internet Domain Name Registration Services) describes the valid subnets & subnet masks that can be used on TCP/IP networks.

Network classes

Internet addresses are allocated by the InterNIC, the organization that administers the Internet. These IP. addresses are divided inkhổng lồ classes. The most common of them are classes A, B, và C. Classes D & E exist, but aren"t used by end users. Each of the address classes has a different mặc định subnet mask. You can identify the class of an IPhường address by looking at its first octet. Following are the ranges of Class A, B, & C Internet addresses, each with an example address:

Class A networks use a default subnet mask of & have 0-127 as their first octet. The address is a class A address. Its first octet is 10, which is between 1 & 126, inclusive sầu.

Class B networks use a default subnet mask of và have sầu 128-191 as their first octet. The address is a class B address. Its first octet is 172, which is between 128 & 191, inclusive sầu.

Class C networks use a mặc định subnet mask of & have sầu 192-223 as their first octet. The address is a class C address. Its first octet is 192, which is between 192 và 223, inclusive.

In some scenartiện ích ios, the default subnet mask values don"t fit the organization needs for one of the following reasons:

The physical topology of the networkThe numbers of networks (or hosts) don"t fit within the mặc định 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. It becomes necessary as you reconcile the logical address scheme of the Internet (the abstract world of IPhường addresses & subnets) with the physical networks in use by the real world.

A system administrator who is allocated a bloông chồng of IP. addresses may be administering networks that aren"t organized in a way that easily fits these addresses. For example, you have sầu 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 (For illustration, this address is actually from a range that isn"t allocated on the Internet.) It means that you can use the addresses to lớn for your 150 hosts.

Two addresses that can"t be used in your example are và because binary addresses with a host portion of all ones và all zeros are invalid. The zero address is invalid because it"s used to lớn specify a network without specifying a host. The 255 address (in binary notation, a host address of all ones) is used khổng lồ broadcast a message khổng lồ every host on a network. Just remember that the first và last address in any network or subnet can"t be assigned to any individual host.

You should now be able khổng lồ give IPhường. addresses to lớn 254 hosts. It 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 inlớn subnets that enable you to use one blochồng of addresses on multiple physical networks.

In this case, you divide your network into lớn four subnets by using a subnet mask that makes the network address larger và the possible range of host addresses smaller. In other words, you are "borrowing" some of the bits used for the host address, and using them for the network portion of the address. The subnet mask gives you four networks of 62 hosts each. It works because in binary notation, 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) và 11000000 (192). (Some administrators will only use two of the subnetworks using as a subnet mask. For more information on this topic, see RFC 1878.) In these four networks, the last six binary digits can be used for host addresses.

Using a subnet mask of, your network then becomes the four networks,, & These four networks would have sầu as valid host addresses:

Rethành viên, again, that binary host addresses with all ones or all zeros are invalid, so you can"t use addresses with the last octet of 0, 63, 64, 127, 128, 191, 192, or 255.

You can see how it works by looking at two host addresses, and If you used the default Class C subnet mask of, both addresses are on the network. However, if you use the subnet mask of, they are on different networks; is on the network, is on the network.

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Default gateways

If a TCP/IP computer needs lớn communicate with a host on another network, it will usually communicate through a device called a router. In TCP/IPhường. terms, a router that is specified on a host, which link the host"s subnet to lớn other networks, is called a mặc định gateway. This section explains how TCP/IPhường determines whether or not khổng lồ skết thúc packets khổng lồ its mặc định gateway to lớn reach another computer or device on the network.

When a host attempts lớn communicate with another device using TCP/IPhường, it performs a comparison process using the defined subnet mask và the destination IP address versus the subnet mask and its own IPhường. 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 sover the packet on the local subnet. If the result of the comparison determines the destination lớn be a remote host, then the computer will forward the packet to the mặc định gateway defined in its TCP/IP properties. It"s 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/IPhường configuration affect network operations, you can solve sầu many comtháng TCP/IPhường 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 lớn some nearby networks but not khổng lồ distant ones. As an example, if you create four subnets (such as in the subnetting example) but use the incorrect subnet mask of in your TCP/IPhường configuration, hosts won"t be able lớn determine that some computers are on different subnets than their own. In this situation, packets destined for hosts on different physical networks that are part of the same Class C address won"t be sent lớn a default gateway for delivery. A comtháng symptom of this issue is when a computer can communicate with hosts that are on its local network and can talk khổng lồ all remote networks except those networks that are nearby và 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 IPhường. Address: If you put computers with IPhường addresses that should be on separate subnets on a local network with each other, they won"t be able lớn communicate. They"ll try lớn sover packets lớn each other through a router that can"t forward them correctly. A symptom of this problem is a computer that can talk lớn hosts on remote networks, but can"t communicate with some or all computers on their local network. To correct this problem, make sure all computers on the same physical network have sầu 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 can communicate with hosts on its own network segment. But it will fail lớn communicate with hosts on some or all remote networks. A host can communicate with some remote networks but not others if the following conditions are true:

A single physical network has more than one router.The wrong router is configured as a default gateway.

This problem is comtháng if an organization has a router khổng lồ an internal TCP/IP network và another router connected to lớn the Internet.


Two popular references on TCP/IP are:

"TCP/IP Illustrated, Volume 1: The Protocols," Richard Stevens, Addison Wesley, 1994"Internetworking with TCP/IPhường, Volume 1: Principles, Protocols, and Architecture," Douglas E. Comer, Prentice Hall, 1995

It is recommended that a system administrator responsible for TCP/IP networks have sầu 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/IPhường network.

Internet--The global collection of networks that are connected together và nội dung a comtháng 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/IPhường. network or the Internet.

IPhường 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 IPhường 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 IPhường address. They have sầu 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 khổng lồ distinguish the network và host portions of an IP. address.

Subnet or Subnetwork--A smaller network created by dividing a larger network into lớn equal parts.

TCP/IP--Used broadly, the mix of protocols, standards, & utilities commonly used on the Internet và 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 large WAN.