IP addressing

Each TCP/IP host is identified by a logical IP address. This address is unique for each host that communicates by using TCP/IP. Each 32-bit IP address identifies a location of a host system on the network in the same way that a street address identifies a house on a city street.

Just as a street address has a standard two-part format (a street name and a house number), each IP address is separated internally into two parts—a network ID and a host ID:

The network ID, also known as a network address, identifies a single network segment within a larger TCP/IP internetwork (a network of networks). All the systems that attach and share access to the same network have a common network ID within their full IP address. This ID is also used to uniquely identify each network within the larger internetwork.

The host ID, also known as a host address, identifies a TCP/IP node (a workstation, server, router, or other TCP/IP device) within each network. The host ID for each device identifies a single system uniquely within its own network.

Here is an example of a 32-bit IP address:

10000011 01101011 00010000 11001000

To make IP addressing easier, IP addresses are expressed in dotted decimal notation. The 32-bit IP address is segmented into four 8-bit octets. The octets are converted to decimal (base-10 numbering system) and separated by periods. Therefore, the previous IP address example is 131.107.16.200 when converted to dotted decimal notation.

The following illustration shows a sample view of an IP address (131.107.16.200) as it is divided into network and host ID sections. The network ID portion (131.107) is indicated by the first two numbers of the IP address. The host ID portion (16.200) is indicated by the last two numbers of the IP address.

Notes

Because IP addresses identify devices on a network, a unique IP address must be assigned to each device on the network.

In general, most computers have only a single network adapter installed and therefore require only a single IP address. If a computer has multiple network adapters installed, each adapter needs its own IP address.

IP address classes

The Internet community has defined five address classes. Class A, B, and C addresses are used for assignment to TCP/IP nodes.

The class of address defines which bits are used for the network and host ID parts of each address. The address class also defines how many networks and hosts per network can be supported.

The following table uses w.x.y.z to designate the four octet values in any given IP address. The table is used to show:

How the value of the first octet (w) of any given IP address effectively indicates the class of address.

How the octets in an address are divided into network ID and host ID.

The number of possible networks and hosts per network available for each class.

Class	Value of w	Network ID	Host ID	Number of networks	Number of hosts per network
A	1–126	w	x.y.z	126	16,777,214
B	128–191	w.x	y.z	16,384	65,534
C	192–223	w.x.y	z	2,097,152	254
D	224–239	Reserved for multicast addressing	N/A	N/A	N/A
E	240–254	Reserved for experimental use	N/A	N/A	N/A

Subnet masks

Network IDs and host IDs within an IP address are distinguished by using a subnet mask. Each subnet mask is a 32-bit number that uses consecutive bit groups of all ones (1) to identify the network ID and all zeroes (0) to identify the host ID portions of an IP address.

For example, the subnet mask normally used with the IP address 131.107.16.200 is the following 32-bit binary number:

11111111 11111111 00000000 00000000

This subnet mask number is 16 one-bits followed by 16 zero-bits, indicating that the network ID and host ID sections of this IP address are both 16 bits in length. Normally, this subnet mask is displayed in dotted decimal notation as 255.255.0.0.

The following table displays subnet masks for the Internet address classes.

Address class	Bits for subnet mask	Subnet mask
Class A	11111111 00000000 00000000 00000000	255.0.0.0
Class B	11111111 11111111 00000000 00000000	255.255.0.0
Class C	11111111 11111111 11111111 00000000	255.255.255.0

Typically, default subnet mask values (as shown in the previous table) are acceptable for most networks with no special requirements and where each IP network segment corresponds to a single physical network.

In some cases, you can use customized subnet masks to implement IP subnetting. With IP subnetting, you can subdivide the default host ID portion of an IP address to specify subnets, which are subdivisions of the original class-based network ID.

By customizing the subnet mask length, you can reduce the number of bits that are used for the actual host ID. For more information about using a custom subnet mask for subnetting your network, see the Windows 2000 Resource Kit.

Important To prevent addressing and routing problems, you should make sure all TCP/IP computers on any network segment use the same subnet mask.

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