What Is an IP Address?

On a TCP/IP network, each device (computer, router, or other device with a connection to the network) is referred to as a host. Each ’I'CP/lP host is Identified by a logical IP address that identifies a computer‘s location on the network in much the same way as a street address identities a house on a Street. Microsoft’s implementation of TCP/IP  enables a TCP/IP host to use a static Internet Protocol (IP) address or to obtain an IP address automatically from a Dynamic Host Configuration Protocol (DHCP) server.For simple network configurations based on local area networks (LANS), Windows 10 also supports automatic assignment of IP addresses. Windows 10 includes many tools that you can use to troubleshoot TCP/IP and test connectivity.

What Is an IP Address?

Every interface on a TCP/IP network is given a unique IP address that identifies it on that network. IP handles this addressing, defining how the addresses are constructed and how packets are routed using those addresses.

An IP address consists of a set of four numbers, each of which can range from 0 to 255. Each of these numbers is separated from the others by a decimal point, so a typical IP address in decimal form might look something like 192.168.1.102. The reason that each number ranges only up to 255 is that each number is actually based on a binary octet, or an eight-digit binary number. The IP address 192.168.1.102 represented in binary form is 11000000 10101000 00000001 01100110. Computers work with the binary format, but it is much easier for people to work with the decimal representation.

An IP address consists of two distinct portions:

• The network ID is a portion of the IP address starting from the left that identifies the network segment on which a host is located. Using the example 192.168.1.102, the portion 192.168.1 might be the network ID. When representing a network if). it is customary to fill in the missing octets with zeroes. So, the proper network ID would be 192168.1.0.

• The host ID is the portion of the IP address that identifies a particular host on a network segment. The host ID for each host must be unique within the network ID. Continuing the example of the IP address 192.168.1.102 (where 192.168.].0 is the network ID), the host ID is 102.

Two computers with different network IDs can have the same host ID. However, the combination of the network ID and the host ID must be unique to all computers in communication with each other.

Hosts depend on a second number called a subnet mask to help determine which portion of an IP address is the network ID and which portion is the host 1D. The subnet mask defines where the network ID stops and the host ID starts. It is easier to see why this works if you step away from the decimal representation for a moment and look at the numbers in their binary format.

 Depicts a single IP address shown in both decimal and binary format. A subnet mask is also shown in both formats. In binary format, a subnet mask always represents a string of unbroken ones followed by a string of unbroken zeroes. The position of the change from ones to zeroes indicates the division of network ID and host ID in an IP address.

Decimal                                                                  Binary

IP Address 135.109.15.42                 10000111 01101101 00001111 00101010
Subnet Mask 255.255.0.0                   11111111 11111111 00000000 00000000

Network ID 135.109.0.0                     10000111 01101101 00000000 00000000
HostlD 0.0.15.42                                00000000 00000000 00001111 00101010

Classful IP Addressing

IP addresses are organized into classes that help define the size of the network being addressed, a system referred to as classful IP addressing. Five different classes of IP addresses define different-sized networks that are capable of holding varying numbers of hosts.

Class A, Network ID is w.0.0.0 with 1-126 range of first octet, 126 available network segments and 16,777,214 available hosts ID a 255.0.0.0 subnet mask
Class B, Network ID is w.x.0.0 with 128-191 range of first octet, 16384 available network segments and 65,534 available hosts ID a 255.255.0.0 subnet mask
Class C, Network ID is w.x.y.0 with 192-223 range of first octet, 2,097,152 available network segments and 254 available hosts ID a 255.255.255.0 subnet mask
Class D, Network ID is N/A with 224-239,N/A range of first octet, N/A available network segments and N/A available hosts ID a N/A subnet mask
Class E, Network ID is N/A with 240-255 range of first octet, N/A available network segments and N/A available hosts ID a N/A subnet mask
Classes A, B, and C are available for registration by public organizations. Actually, mo“ of these addresses were snapped up long ago by major companies and Internet service providers (ISPs), so the actual assignment of an IP address to your organization Will likely come from your chosen ISP. Classes D and E are reserved for special use.

The address class determines the subnet mask used, and therefore determines the lel. sion between the network ID and the host ID. For class A, the network 1D is the film octet in the IP address (for example, the 98 in the address 98.162.102.53 is the network ID). For class B, it is the first two octets; and for class C, it is the first three octets. The remaining octets not used by the network ID identify the host ID.

  Remember the IP address ranges that fall into each class and the default sub net mask for each class. This information not only helps to determine how a classful lP addressing scheme will apply to a situation. but also how to customize a scheme using the classless method (discussed next).

Classless lnterdomain Routing (CIDR)

In the classful method of IP addressing, the number of networks and hosts available for a specific address class is predetermined by the default subnet mask for the class. As a result, an organization that is allocated a network ID has a single fixed network ID and a specific number of hosts. With the single network ID, the organization can have only one network connecting its allocated number of hosts. If the number of hosts is large, the network cannot perform efficiently. To solve this problem, the concept of classless interdomain routing (CIDR) was introduced.

CIDR allows a single classful network ID to be divided into smaller network IDs. The idea is that you take the default subnet mask used for the class to which your IP address range belongs, and then borrow some of the bits used for the host ID to use as an extension to the network ID, creating a custom subnet mask.
A custom subnet mask is not restricted by the same rules used ln the clamsful method. Remember that a subnet mask consists of a set of four numbers, similar to an 11’ address. Consider the default subnet mask for a class B network (255.255.0.0), which m binary format would be the following:

11111111 11111111 00000000 00000000

This mask specifies that the first 16 bits of an IP address are to be used for the network ID and the second 16 bits are to be used for the host 1D. To create a custom subnet mask, you would just extend the mask into the host ID ponion. However, you must extend this by adding ones from left to right. Remember that a subnet mask must be an unbroken string of ones followed by an unbroken string of zeroes. For example, a custom subnet mask might look like this:

11111111 11111111 11111000 00000000

The value 11111000 in decimal format would be 248, making this IP address 255.255.248.0. T


Binary Value    Decimal Value
10000000                128
11000000                192
11100000                224
11110000                240
11111000                248
11111100                252
11111110                 254

In the classful method, each of the four numbers in a subnet mask can be only the maximum value 255 or the minimum value 0. The four numbers are then arranged as contiguous octets of 255, followed by contiguous octets of 0. For example, 255.255.0.0 is a valid subnet mask, whereas 255.0.255.0 is not. The 255 octets identify the network ID, and the 0 octets identify the host ID. For example, the subnet mask 255.255.0.0 identifies the network ID as the first two numbers in the IP address.

When subnetting an existing network ID to create additional subnets, you can use any of the preceding subnet masks with any IP address or network ID. So the IP address 184.12.102.2 could have the subnet mask 255.255.255.0 and network ID 184.12.102.0. as opposed to the default subnet mask 255,255.0.0 with the network ID 184.12.0.0. This allows an organization to subnet an existing class B network ID of 184.12.0.0 into smaller subnets to match the actual configuration of their network.

Classful Addtesslng and CIDR

Although classful IP addressing is important to understand, it is primarilying only from a historical perspective. Most modern networks that use class A or B addresses are no longer organized by using the traditional classful subnet mask. Originally, routers and routing protocols did not separately network IDs and subnet masks because memory for these devices was scam expensive. Instead, classful routing was necessary because devices had to assume the subnet mask based on the first octet. Today, memory is cheap, and router (and routing protocol) stores both network IDs and subnet masks in the routing tables.

Private Addressing

Every network interface that is connected directly to the Internet must have an 1]) address registered with the Internet Assigned Numbers Authority (IANA), which prevents IP address conflicts between devices. If you are configuring a private network that is not connected to the Internet or one that exists behind a firewall or proxy server, you can configure devices on your network with private addresses and have only the public address configured on the interface that is visible to the Internet.

Each address class has a range of private addresses available for general use:

• Class A: 10.0.0.0 through 10.255.255.255 
• Class B: 172.16.0.0 through 172.31.255.255 
• Class C: 192.168.0.0 through 192.168.255.255

You can choose whichever range you like to use for your network and implement custom subnets as you see fit. None of these addresses is ever officially assigned to a publicly accessible Internet host.