IPv4 Addressing

• IPv4 addresses are 32-bit addresses
• IPv4 addresses uniquely and universally define a device's Internet connection
• Each IPv4 address defines one, and only one, connection to the Internet
• Two devices on the Internet cannot have the same address simultaneously
• Addresses can be assigned to a device for a time period and then reassigned

Address Space

• Address space is the total number of addresses used by a protocol
• A protocol using N bits to define an address has an address space of 2^N
• IPv4 employs 32-bit addresses, resulting in an address space of 2^32, or 4,294,967,296 (more than 4 billion)
• Theoretically, IPv4 could connect over 4 billion devices to the Internet

Notations

• IPv4 addresses are shown in binary and dotted decimal notations

Binary Notation

• Binary notation displays the IPv4 address as 32 bits
• Each octet is often referred to as a byte
• IPv4 address is often referred to as a 32-bit address or a 4-byte address

Dotted-Decimal Notation

• Dotted-decimal notation represents Internet addresses in decimal form, with bytes separated by dots
• In dotted-decimal notation, each number ranges from 0 to 255 because each byte (octet) is 8 bits

Classful Addressing

• IPv4 addressing originally used classes, known as classful addressing
• Classful addressing divides the address space into five classes: A, B, C, D, and E
• Each class occupies a part of the address space
• The address class can be determined from the first few bits in binary notation
• The address class can be determined from the first byte in dotted-decimal notation

Classes and Blocks

• Classful addressing divides each class into a fixed number of blocks, all having a fixed size
• Organizations were granted addresses in class A, B, or C
• Class A was designed for large organizations
• Class B was designed for midsize organizations
• Class C was designed for small organizations
• Class A blocks were too large for most organizations, leading to wasted addresses
• Class B blocks were also too large for many organizations
• Class C blocks were too small for many organizations
• Class D addresses were intended for multicasting
• Class E addresses were reserved for future use, resulting in wasted addresses

Netid and Hostid

• In classful addressing, IP addresses are divided into netid and hostid
• Netid and hostid lengths vary depending on the address class
• Classes D and E do not fit the netid and hostid concept
• Class A uses one byte for the netid and three bytes for the hostid
• Class B uses two bytes for the netid and two bytes for the hostid
• Class C uses three bytes for the netid and one byte for the hostid
• A mask (or default mask) can also be used, which is a 32-bit number of contiguous 1s, followed by contiguous 0s
• Classes D and E do not fit the mask concept
• A class A address mask starts with eight 1 bits that defines a netid; the next 24 bits defines the hostid
• Slash notation, or Classless Interdomain Routing (CIDR) notation, can also be applied to classful addressing

Subnetting

• During classful addressing, subnetting divides a large class A or B block into smaller networks (subnets)
• Subnetting increases the number of 1s in the mask

Supernetting

• Supernetting emerged when class A and B addresses were depleted and there was a demand for midsize blocks
• Supernetting combines several class C blocks to create a larger address range
• Supernetting decreases the number of 1s in the mask
• Classless addressing eliminated the need for supernetting

Address Depletion

• Flaws in classful addressing and rapid Internet growth led to near address depletion
• The number of devices on the Internet is less than the 2^32 address space
• Class A and B addresses are in short supply, and class C blocks are too small

Classless Addressing

• Classless addressing was designed to address depletion and provide more organizations Internet access
• Classless addressing grants addresses in blocks
• Block sizes vary based on the entity's nature and size

Address Blocks

• In classless addressing, entities are granted address blocks (ranges)
• Block sizes depend on the entity's nature and size
• A household might receive two addresses
• Large organizations might receive thousands of addresses
• An ISP is given thousands or hundreds of thousands of addresses, based on the number of customers it may serve
• To simplify address handling, three restrictions are imposed on classless address blocks

Address Block Restrictions

• Addresses within a block must be contiguous
• The number of addresses in a block must be a power of 2
• The first address must be evenly divisible by the number of addresses
• The address block restrictions apply to [205.16.37.32-205.16.37.47]
• The 16 addresses are contiguous
• Since 16 = 2^4, the address is a power of 2
• The number 3,440,387,360 is divisible by 16

Mask

• A mask combined with any address in the block can define a block of addresses
• A mask is a 32-bit number with n leftmost bits as 1s and 32 – n rightmost bits as 0s
• In classless addressing, the mask for a block can range from 0 to 32
• Using the /n value, also known as CIDR notation, with the address completely defines the block

First Address

• The first address in a block can be found by setting the rightmost 32 - n bits in the binary notation of the address to 0s.

Network Addresses

• A key concept in IP addressing is the network address
• When an organization receives an address block, it can allocate those addresses to devices
• The first address is used as the network address, defining the organization network
• Routers use this address to direct messages to the organization from the outside
• The organization network is connected to the Internet via a router, which has two addresses
• One router address belongs to the granted block, while the other belongs to the external network, that is labeled x.y.z.t/n
• All messages destined for addresses between designated numbers, such as 205.16.37.32 to 205.16.37.47, are sent directly or indirectly to x.y.z.t/n

Two-Level Hierarchy: No Subnetting

• An IP address, without subnetting, can define only two hierarchy levels
• The leftmost bits of the address define the network, while the rightmost bits define the host

Prefixes and Suffixes

• Common prefixes and suffixes are used in the two-level hierarchy system
• The prefix is the part of the address that defines the network
• The suffix is the part of the address that defines the host

Three-Levels of Hierarchy: Subnetting

• An organization with a large address block might create network clusters (subnets) and divide addresses among them
• The organization is still viewed as one entity from the outside, while there are subnets internally
• The router sends all messages to the appropriate subnets
• The organization creates subblocks of addresses, each assigned to specific subnets, and assigns each subnet its own mask

Subnetting Example

• A mask for an organization given the block 17.12.14.0/26, that contains 64 addresses and splits into three offices requires three subblocks of 32, 16, and 16 addresses to assign to the subnets

• In subnet 1, to have 32 addresses, 2^(32-n1) must be 32, therefore the mask n1 is 27

• In subnet 2, to have 16 addresses, 2^(32-n2) must be 16, therefore the mask n2 is 28

• In subnet 3, to have 16 addresses, 2^(32-n3) must be 16, therefore the mask n3 is 28

• With an organization mask of 26, there are masks of 27, 28 and 28

• In subnet 1, the address 17.12.14.29/27 uses the mask/27, resulting in the subnet address of 17.12.14.0

• Host: 00010001 00001100 00001110 00011101

• Mask: /27

• Subnet: 00010001 00001100 00001110 00000000 (17.12.14.0)

• In subnet 2, the address 17.12.14.45/28 uses the mask/28, resulting in the subnet address of 17.12.14.32

• Host: 00010001 00001100 00001110 00101101

• Mask: /28

• Subnet: 00010001 00001100 00001110 00100000 (17.12.14.32)

• In subnet 3, the address 17.12.14.50/28 uses the mask/28, resulting in the subnet address of 17.12.14.48

• Host: 00010001 00001100 00001110 00110010

• Mask: /28

• Subnet: 00010001 00001100 00001110 00110000 (17.12.14.48)

Multiple Levels of Hierarchy

• Classless addressing allows unlimited hierarchical levels
• Organizations can divide address blocks into subblocks, which can be further divided
• A national ISP can divide a block into smaller blocks for regional ISPs
• A regional ISP can divide its block for local ISPs
• A local ISP can divide its block for specific organizations
• An organization can divide its block into subnets

Address Allocation

• A key challenge of classless addressing is address allocation
• The Internet Corporation for Assigned Names and Addresses (ICANN) has the ultimate responsibility for address allocation
• ICANN assigns large blocks to ISPs, which then divide them into subblocks, and grants the subblocks to the ISP's customers, or Internet users.
• This is defined as address aggregation, or the combination of several address blocks granted to one ISP

Address Aggregation Example

• To distribute its allocation of the block of addresses starting with 190.100.0.0/16 (65,536 addresses) to three groups of customers that make up an ISPs customer base
  • The first group has 64 customers, each needing 256 addresses
  • The second group has 128 customers, each needing 128 addresses
  • The third group has 128 customers, each needing 64 addresses

Resulting Design

• Group 1 Design: 8 bits (log2 256) are needed to define each host with a prefix length of 32-8 = 24
  • 1st Customer: 190.100.0.0/24 to 190.100.0.255/24
  • 2nd Customer: 190.100.1.0/24 to 190.100.1.255/24
  • 64th Customer: 190.100.63.0/24 to 190.100.63.255/24
  • Total = 16,385
• Group 2 Design: 7 bits (log2 128) are needed to define each host with a prefix length of 32-7 = 25
  • 1st Customer: 190.100.64.0/25 to 190.100.64.127/25
  • 2nd Customer: 190.100.64.128/25 to 190.100.64.255/25
  • 128 Customer: 190.100.127.128/25 to 190.100.127.255/25
  • Total = 16385
• Group 3: 6 bits (log264) are needed to define each host with a prefix length of 32-6 = 26
  • 1st Customer: 190.100.128.0/26 to 190.100.128.63/26
  • 2nd Customer: 190.100.128.64/26 to 190.100.128.127/26
  • 128 Customer: 190.100.159.192/26 to 190.100.159.255/26
  • Total = 8192
• 65,536 addresses granted to the ISP
• 40,960 addresses were allocated by the ISP
• 24,576 addresses were available