Like this article? We recommend CCNA candidates need to be fluent in their understanding of IP addressing concepts. The following sections detail how IP addresses are organized and analyzed, with a view to answering subnetting questions. Show Address ClassEarly in the development of IP, the IANA (Internet Assigned Numbers Authority) designated five classes of IP address: A, B, C, D, and E. These classes were identified based on the pattern of high-order bits (the high-value bits at the beginning of the first octet). The result is that certain ranges of networks are grouped into classes in a pattern based on the binary values of those high-order bits, as detailed in Table 3.7: Table 3.7 Address Class and Range
You might notice that 127 is missing. This is because at some point the address 127.0.0.1 was reserved for the loopback (sometimes called "localhost") IP—this is the IP of the TCP/IP protocol itself on every host machine. Default Subnet MaskEach class of address is associated with a default subnet mask, as shown in Table 3.8. An address using its default mask defines a single IP broadcast domain—all the hosts using that same network number and mask can receive each other’s broadcasts and communicate via IP. Table 3.8 Address Class and Default Masks
One of the rules is that a subnet mask must be a contiguous string of 1s followed by a contiguous string of 0s. There are no exceptions to this rule: A valid mask is always a string of 1s, followed by 0s to fill up the rest of the 32 bits. Therefore, the only possible valid values in any given octet of a subnet mask are 0, 128, 192, 224, 240, 248, 252, 254, and 255. Any other value is invalid. The Network FieldEvery IP address is composed of a network component and a host component. The subnet mask has a single purpose: to identify which part of an IP address is the network component and which part is the host component. Look at a 32-bit IP address expressed in binary, with the subnet mask written right below it. Figure 3.1 shows an example.
 Figure 3.1 IP address and mask in binary, showing network and host fields. Anywhere you see a binary 1 in the subnet mask, it means "the matching bit in the IP address is part of the network component." In this example, the network part of the address is 192.168.0.X, and the last octet (X) will be the host component. Because there are 24 bits in a row in the mask, we can also use a shortcut for the mask notation of /24. These examples show how a dotted decimal mask can be expressed in slash notation: 192.168.1.66 255.255.255.0 = 192.168.1.66 /24 172.16.0.12 255.255.0.0 = 172.16.0.12 /16 10.1.1.1 255.0.0.0 = 10.1.1.1 /8 This slash notation is sometimes called CIDR (Classless Inter-Domain Routing) notation. For some reason, it’s a concept that confuses students, but honestly it’s the easiest concept of all: The slash notation is simply the number of 1s in a row in the subnet mask. The real reason to use CIDR notation is simply that it is easier to say and especially to type—and it appears interchangeably with dotted decimal throughout the exam. Every IP address has a host component and a network component, and the 1s in the mask tell us which bits in the address identify the network component. The Host FieldIf the 1s in the mask identify the network component of an address, the 0s at the end of the mask identify the host component. In the preceding example, the entire last octet is available for the host IP number. The number of 0s at the end of the mask mathematically define how many hosts can be on any given network or subnet. The 1s in the mask always identify the network component, and the 0s at the end of the mask always identify the host component of any IP address. Non-default MasksAt this point, you should be able to recognize what class an address belongs to, and what its default mask is supposed to be. Here’s the big secret: If a mask is longer than it is supposed to be, that network has been subnetted. So it is clearly another critical skill that you be able to spot those non-default masks. The Subnet FieldBecause we have extended the subnet mask past the default boundary into the bits that were previously host bits, we identify the bits we "stole" from the host part as the subnet field. The subnet field is relevant because those bits mathematically define how many subnets we create. Figure 3.2 uses the same IP address from our previous example, but now we have applied a subnetted mask that is longer than the default. Note that this creates the subnet field.
Figure 3.2 IP address and non-default mask in binary illustrating the subnet field. Figure 3.2 identifies the two extra bits past the default boundary as the subnet field—they used to be in the host field, but we subnetted and stole them to become the subnet field.
This article is intended as a general introduction to the concepts of Internet Protocol (IP) networks and subnetting. A glossary is included at the end of article. Applies to: Windows 10 - all editions When you configure the TCP/IP protocol on a Windows computer, the TCP/IP configuration settings require:
To configure TCP/IP correctly, it's necessary to understand how TCP/IP networks are addressed and divided into networks and subnetworks. 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 them 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 address and see how it's organized. IP addresses: Networks and hostsAn IP address is a 32-bit number. It 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 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 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 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 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 192.168.123. Network .132 Host or 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 aren't fixed. Unless you have more information, the network and host addresses above can't be determined. This information is supplied in another 32-bit number called a subnet mask. The subnet mask is 255.255.255.0 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 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) 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 (192.168.123.0) 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:
Internet RFC 1878 (available from InterNIC-Public Information Regarding Internet Domain Name Registration Services) describes the valid subnets and subnet masks that can be used on TCP/IP networks. Network classesInternet addresses are allocated by the InterNIC, the organization that administers the Internet. These IP addresses are divided into classes. The most common of them are classes A, B, and C. Classes D and E exist, but aren't 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:
In some scenarios, the default subnet mask values don't fit the organization needs for one of the following reasons:
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 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 aren't 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 isn't allocated on the Internet.) It means that you can use the addresses 192.168.123.1 to 192.168.123.254 for your 150 hosts. Two addresses that can't 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's 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 can't be assigned to any individual host. You should now be able to give IP addresses to 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 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 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. It 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 six 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 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, 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 gatewaysIf 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 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's then the responsibility of the router to forward the packet to the correct subnet. TroubleshootingTCP/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 won't be able to 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 to a default gateway for delivery. A common symptom of this issue is when a computer can communicate with hosts that are on its local network and can talk to all remote networks except those networks 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 won't be able to communicate. They'll try to send packets to each other through a router that can't forward them correctly. A symptom of this problem is a computer that can talk to 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 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 to 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:
This problem is common if an organization has a router to an internal TCP/IP network and another router connected to the Internet. ReferencesTwo popular references on TCP/IP are:
It is recommended that a system administrator responsible for TCP/IP networks have at least one of these references available. Glossary
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