Understanding IPv4 Addressing

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IPv4 addresses are 32-bit numbers that are typically displayed in dotted decimal notation. A 32-bit address contains two primary parts: the network prefix and the host number.

All hosts within a single network share the same network address. Each host also has an address that uniquely identifies it. Depending on the scope of the network and the type of device, the address is either globally or locally unique. Devices that are visible to users outside the network (webservers, for example) must have a globally unique IP address. Devices that are visible only within the network must have locally unique IP addresses.

IP addresses are assigned by a central numbering authority called the Internet Assigned Numbers Authority (IANA). IANA ensures that addresses are globally unique where needed and has a large address space reserved for use by devices not visible outside their own networks.

IPv4 Classful Addressing

To provide flexibility in the number of addresses distributed to networks of different sizes, 4-octet (32-bit) IP addresses were originally divided into three different categories or classes: class A, class B, and class C. Each address class specifies a different number of bits for its network prefix and host number:

·         Class A addresses use only the first byte (octet) to specify the network prefix, leaving 3 bytes to define individual host numbers.

·         Class B addresses use the first 2 bytes to specify the network prefix, leaving 2 bytes to define host addresses.

·         Class C addresses use the first 3 bytes to specify the network prefix, leaving only the last byte to identify hosts.

In binary format, with an x representing each bit in the host number, the three address classes can be represented as follows:

00000000 xxxxxxxx xxxxxxxx xxxxxxxx (Class A)

00000000 00000000 xxxxxxxx xxxxxxxx (Class B)

00000000 00000000 00000000 xxxxxxxx (Class C)

Because each bit (x) in a host number can have a 0 or 1 value, each represents a power of 2. For example, if only 3 bits are available for specifying the host number, only the following host numbers are possible:

111 110 101 100 011 010 001 000

In each IP address class, the number of host-number bits raised to the power of 2 indicates how many host numbers can be created for a particular network prefix. Class A addresses have 2²⁴ (or 16,777,216) possible host numbers, class B addresses have 2¹⁶ (or 65,536) host numbers, and class C addresses have 2⁸ (or 256) possible host numbers.

IPv4 Dotted Decimal Notation

The 32-bit IPv4 addresses are most often expressed in dotted decimal notation, in which each octet (or byte) is treated as a separate number. Within an octet, the rightmost bit represents 2⁰ (or 1), increasing to the left until the first bit in the octet is 2⁷ (or 128). Following are IP addresses in binary format and their dotted decimal equivalents:

11010000 01100010 11000000 10101010 = 208.98.192.170

01110110 00001111 11110000 01010101 = 118.15.240.85

00110011 11001100 00111100 00111011 = 51.204.60.59

IPv4 Subnetting

Because of the physical and architectural limitations on the size of networks, you often must break large networks into smaller subnetworks. Within a network, each wire or ring requires its own network number and identifying subnet address.

Figure 1 shows two subnets in a network.

Figure 1: Subnets in a Network

Figure 1 shows three devices connected to one subnet and three more devices connected to a second subnet. Collectively, the six devices and two subnets make up the larger network. In this example, the network is assigned the network prefix 192.14.0.0, a class B address. Each device has an IP address that falls within this network prefix.

In addition to sharing a network prefix (the first two octets), the devices on each subnet share a third octet. The third octet identifies the subnet. All devices on a subnet must have the same subnet address. In this case, the alpha subnet has the IP address 192.14.126.0 and the beta subnet has the IP address 192.14.17.0.

The subnet address 192.14.17.0 can be represented as follows in binary notation:

11000000 . 00001110 . 00010001 . xxxxxxxx

Because the first 24 bits in the 32-bit address identify the subnet, the last 8 bits are not significant. To indicate the subnet, the address is written as 192.14.17.0/24 (or just 192.14.17/24). The /24 is the subnet mask (sometimes shown as 255.255.255.0).

IPv4 Variable-Length Subnet Masks

Traditionally, subnets were divided by address class. Subnets had either 8, 16, or 24 significant bits, corresponding to 2²⁴, 2¹⁶, or 2⁸ possible hosts. As a result, an entire /16 subnet had to be allocated for a network that required only 400 addresses, wasting 65,136 (2¹⁶ – 400 = 65,136) addresses.

To help allocate address spaces more efficiently, variable-length subnet masks (VLSMs) were introduced. Using VLSM, network architects can allocate more precisely the number of addresses required for a particular subnet.

For example, suppose a network with the prefix 192.14.17/24 is divided into two smaller subnets, one consisting of 18 devices and the other of 46 devices.

To accommodate 18 devices, the first subnet must have 2⁵ (32) host numbers. Having 5 bits assigned to the host number leaves 27 bits of the 32-bit address for the subnet. The IP address of the first subnet is therefore 192.14.17.128/27, or the following in binary notation:

 11000000 . 00001110 . 00010001 . 100xxxxx

The subnet mask includes 27 significant digits.

To create the second subnet of 46 devices, the network must accommodate 2⁶ (64) host numbers. The IP address of the second subnet is 192.14.17.64/26, or

 11000000 . 00001110 . 00010001 . 01xxxxxx

By assigning address bits within the larger /24 subnet mask, you create two smaller subnets that use the allocated address space more efficiently.