IPv4 Addressing

Applies To: Windows Server 2008

IPv4 Addressing

For IP version 4, each TCP/IP host is identified by a logical IP address. The IP address is a Network layer address and has no dependence on the Data-Link layer address (such as a MAC address of a network adapter). A unique IP address is required for each host and network component that communicates using TCP/IP and can be assigned manually or by using Dynamic Host Configuration Protocol (DHCP).

The IP address identifies a system’s location on the network in the same way a street address identifies a house on a city block. Just as a street address must identify a unique residence, an IP address must be globally unique to the internetwork and have a uniform format.

Each IP address includes a network ID and a host ID.

• The network ID (also known as a network address) identifies the systems that are located on the same physical network bounded by IP routers. All systems on the same physical network must have the same network ID. The network ID must be unique to the internetwork.

• The host ID (also known as a host address) identifies a workstation, server, router, or other TCP/IP host within a network. The host address must be unique to the network ID.

IPv4 Address Syntax

An IP address consists of 32 bits. Instead of expressing IPv4 addresses 32 bits at a time using binary notation (Base2), it is standard practice to segment the 32 bits of an IPv4 address into four 8-bit fields called octets. Each octet is converted to a decimal number (base 10) from 0–255 and separated by a period (a dot). This format is called dotted decimal notation. The following table provides an example of an IP address in binary and dotted decimal formats.

An IP Address in Binary and Dotted Decimal Formats

For example, the IPv4 address of 11000000101010000000001100011000 is:

• Segmented into 8-bit blocks: 11000000 10101000 00000011 00011000.

• Each block is converted to decimal: 192 168 3 24

• The adjacent octets are separated by a period: 192.168.3.24.

The notation w.x.y.z is used when referring to a generalized IP address, and is shown the following figure.

IP Address

Types of IPv4 Addresses

The Internet standards define the following types of IPv4 addresses:

• Unicast. Assigned to a single network interface located on a specific subnet on the network and used for one-to-one communications.

• Multicast. Assigned to one or more network interfaces located on various subnets on the network and used for one-to-many communications.

• Broadcast. Assigned to all network interfaces located on a subnet on the network and used for one-to-everyone-on-a-subnet communications.

The following sections describe these types of addresses in detail.

IPv4 Unicast Addresses

The IPv4 unicast address identifies an interface’s location on the network in the same way a street address identifies a house on a city block. Just as a street address must identify a unique residence, an IPv4 unicast address must be globally unique to the network and have a uniform format.

Each IPv4 unicast address includes a network ID and a host ID.

• The network ID (also known as a network address) is the fixed portion of an IPv4 unicast address that identifies the set of interfaces that are located on the same physical or logical network segment as bounded by IPv4 routers. A network segment on TCP/IP networks is also known as a subnet. All systems on the same physical or logical subnet must use the same network ID and the network ID must be unique to the entire TCP/IP network.

• The host ID (also known as a host address) is the variable portion of an IPv4 unicast address that is used to identify a network node’s interface on a subnet. The host ID must be unique to the network ID.

If the network ID is unique to the TCP/IP network and the host ID is unique to the network ID, then the entire IPv4 unicast address consisting of the network ID and host ID is unique to the entire TCP/IP network.

IPv4 Multicast Addresses

IPv4 multicast addresses are used for single-packet one-to-many delivery. On an IPv4 multicast-enabled intranet, an IPv4 packet addressed to an IPv4 multicast address is forwarded by routers to the subnets on which there are hosts listening to the traffic sent to the IPv4 multicast address. IPv4 multicast provides an efficient one-to-many delivery service for many types of communication.

IPv4 multicast addresses are defined by the class D Internet address class: 224.0.0.0/4. IPv4 multicast addresses range from 224.0.0.0 through 239.255.255.255. IPv4 multicast addresses for the 224.0.0.0/24 address prefix (224.0.0.0 through 224.0.0.255) are reserved for local subnet multicast traffic.

IPv4 Broadcast Addresses

IPv4 uses a set of broadcast addresses to provide a one-to-everyone on the subnet delivery service. Packets sent to IPv4 broadcast addresses are processed by all the interfaces on the subnet. The following are the different types of IPv4 broadcast addresses:

• Network broadcast. Formed by setting all the host bits to 1 for a classful address prefix. An example of a network broadcast address for the classful network ID 131.107.0.0/16 is 131.107.255.255. Network broadcasts are used to send packets to all interfaces of a classful network. IPv4 routers do not forward network broadcast packets.

• Subnet broadcast. Formed by setting all the host bits to 1 for a classless address prefix. An example of a network broadcast address for the classless network ID 131.107.26.0/24 is 131.107.26.255. Subnet broadcasts are used to send packets to all hosts of a classless network. IPv4 routers do not forward subnet broadcast packets. For a classful address prefix, there is no subnet broadcast address, only a network broadcast address. For a classless address prefix, there is no network broadcast address, only a subnet broadcast address.

• All-subnets-directed broadcast. Formed by setting all the original classful network ID host bits to 1 for a classless address prefix. A packet addressed to the all-subnets-directed broadcast was defined to reach all hosts on all of the subnets of a subnetted class-based network ID. An example of an all-subnets-directed broadcast address for the subnetted network ID 131.107.26.0/24 is 131.107.255.255. The all-subnets-directed broadcast is the network broadcast address of the original classful network ID. IPv4 routers can forward all-subnets directed broadcast packets, however the use of the all-subnets-directed broadcast address is deprecated in RFC 1812.

• Limited broadcast. Formed by setting all 32 bits of the IPv4 address to 1 (255.255.255.255). The limited broadcast address is used for one-to-everyone delivery on the local subnet when the local network ID is unknown. IPv4 nodes typically only use the limited broadcast address during an automated configuration process such as Boot Protocol (BOOTP) or DHCP. For example, with DHCP, a DHCP client must use the limited broadcast address for all traffic sent until the DHCP server acknowledges the use of the offered IPv4 address configuration. IPv4 routers do not forward limited broadcast packets.

Internet Address Classes

The Internet community originally defined address classes to accommodate different types of addresses and networks of varying sizes. The class of address defined which bits were used for the network ID and which bits were used for the host ID. It also defined the possible number of networks and the number of hosts per network. Of five address classes, class A, B, and C addresses were defined for IPv4 unicast addresses. Class D addresses were defined for IPv4 multicast addresses and class E addresses were defined for experimental uses.

Class A

Class A network IDs were assigned to networks with a very large number of hosts. The high-order bit in a class A address is always set to zero, which makes the address prefix for all class A networks and addresses 0.0.0.0/1 (or 0.0.0.0, 128.0.0.0). The next seven bits (completing the first octet) are used to enumerate class A network IDs. Therefore, address prefixes for class A network IDs have an 8-bit prefix length (/8 or 255.0.0.0). The remaining 24 bits (the last three octets) are used for the host ID. The address prefix 0.0.0.0/0 (or 0.0.0.0, 0.0.0.0) is a reserved network ID and 127.0.0.0/8 (or 127.0.0.0, 255.0.0.0) is reserved for loopback addresses. Out of a total of 128 possible class A networks, there are 126 networks and 16,777,214 hosts per network.

The following figure illustrates the structure of class A addresses.

Structure of class A addresses

Class B

Class B network IDs were assigned to medium to large-sized networks. The two high-order bits in a class B address are always set to 10, which makes the address prefix for all class B networks and addresses 128.0.0.0/2 (or 128.0.0.0, 192.0.0.0). The next 14 bits (completing the first two octets) are used to enumerate class B network IDs. Therefore, address prefixes for class B network IDs have a 16-bit prefix length (/16 or 255.255.0.0). The remaining 16 bits (last two octets) are used for the host ID. With 14 bits to express class B network IDs and 16 bits to express host IDs, this allows for 16,384 networks and 65,534 hosts per network.

The following figure illustrates the structure of class B addresses.

Structure of class B addresses

Class C

Class C addresses were assigned to small networks. The three high-order bits in a class C address are always set to 110, which makes the address prefix for all class C networks and addresses 192.0.0.0/3 (or 192.0.0.0, 224.0.0.0). The next 21 bits (completing the first three octets) are used to enumerate class C network IDs. Therefore, address prefixes for class C network IDs have a 24-bit prefix length (/24 or 255.255.255.0). The remaining 8 bits (the last octet) are used for the host ID. With 21 bits to express class C network IDs and 8 bits to express host IDs, this allows for 2,097,152 networks and 254 hosts per network.

The following figure illustrates the structure of class C addresses.

Structure of class C addresses

Class D

Class D addresses are reserved for IPv4 multicast addresses. The four high-order bits in a class D address are always set to 1110, which makes the address prefix for all class D addresses 224.0.0.0/4 (or 224.0.0.0, 240.0.0.0).

Class E

Class E addresses are reserved for experimental use. The high-order bits in a class E address are set to 1111, which makes the address prefix for all class E addresses 240.0.0.0/4 (or 240.0.0.0, 240.0.0.0)

The following table is a summary of the Internet address classes A, B, and C that can be used for IPv4 unicast addresses.

Internet Address Class Summary

Modern Internet Addresses

The Internet address classes are an obsolete unicast address allocation method that proved to be an inefficient way to assign network IDs and addresses to organizations connected to the Internet. For example, a large organization with a class A network ID can have up to 16,777,214 hosts. However, if the organization only uses 70,000 host IDs, then 16,707,214 potential IPv4 unicast addresses for the Internet are wasted.

On the modern-day Internet, IPv4 address prefixes are handed out to organizations based on the organization’s actual need for Internet-accessible IPv4 unicast addresses using a method known as Classless Inter-Domain Routing (CIDR). For example, an organization determines that it needs 2,000 Internet-accessible IPv4 unicast addresses. The Internet Corporation for Assigned Names and Numbers (ICANN) or an Internet service provider (ISP) allocates an IPv4 address prefix in which 21 bits are fixed, leaving 11 bits for host IDs. From the 11 bits for host IDs, the organization can create 2,032 possible IPv4 unicast addresses.

CIDR-based address allocations typically start at 8 bits. The following table lists the required number of host IDs and the corresponding prefix length for CIDR-based address allocations.

Host IDs Needed and CIDR-based Prefix Lengths

Public and Private Addresses

If you want direct (routed) connectivity to the Internet, then you must use public addresses. If you want indirect (proxied or translated) connectivity to the Internet, you can use either public or private addresses. If your intranet is not connected to the Internet in any way, you can use any unicast IPv4 addresses that you want. However, you should use private addresses to avoid network renumbering when your intranet is eventually connected to the Internet.

Public addresses

Public addresses are assigned by ICANN and consist of either historically allocated class-based network IDs or, more recently, CIDR-based address prefixes that are guaranteed to be globally unique on the Internet. For CIDR-based address prefixes, the value of w (the first octet) is in the ranges of 1 through 126 and 128 through 223, with the exception of the private address prefixes described in “Private Addresses.”

When the public addresses are assigned, routes are added to the routers of the Internet so that traffic sent to an address that matches the assigned public address prefix can reach the assigned organization. For example, when an organization is assigned an address prefix in the form of a network ID and prefix length, that address prefix also exists as a route in the routers of the Internet. IPv4 packets destined to an address within the assigned address prefix are routed to the proper destination.

Private addresses

Each IPv4 interface requires an IPv4 address that is globally unique to the IPv4 network. In the case of the Internet, each IPv4 interface on a subnet connected to the Internet requires an IPv4 address that is globally unique to the Internet. As the Internet grew, organizations connecting to the Internet required a public address for each interface on their intranets. This requirement placed a huge demand on the pool of available public addresses.

When analyzing the addressing needs of organizations, the designers of the Internet noted that for many organizations, most of the hosts on an organization’s intranet did not require direct connectivity to the Internet. Those hosts that did require a specific set of Internet services, such as Web access and e-mail, typically access the Internet services through Application layer gateways such as proxy servers and e-mail servers. The result is that most organizations only required a small number of public addresses for those nodes (such as proxies, servers, routers, firewalls, and translators) that were directly connected to the Internet.

For the hosts within the organization that do not require direct access to the Internet, IPv4 addresses that do not duplicate already-assigned public addresses are required. To solve this addressing problem, the Internet designers reserved a portion of the IPv4 address space and named this space the private address space. An IPv4 address in the private address space is never assigned as a public address. IPv4 addresses within the private address space are known as private addresses. Because the public and private address spaces do not overlap, private addresses never duplicate public addresses.

The private address space specified in RFC 1918 is defined by the following address prefixes:

• 10.0.0.0/8 (10.0.0.0, 255.0.0.0)

  Allows the following range of valid IPv4 unicast addresses: 10.0.0.1 to 10.255.255.254. The 10.0.0.0/8 address prefix has 24 host bits that can be used for any addressing scheme within the private organization.

• 172.16.0.0/12 (172.16.0.0, 255.240.0.0)

  Allows the following range of valid IPv4 unicast addresses: 172.16.0.1 to 172.31.255.254. The 172.16.0.0/12 address prefix has 20 host bits that can be used for any addressing scheme within the private organization.

• 192.168.0.0/16 (192.168.0.0, 255.255.0.0)

  Allows the following range of valid IPv4 unicast addresses: 192.168.0.1 to 192.168.255.254. The 192.168.0.0/16 address prefix has 16 host bits that can be used for any addressing scheme within the private organization.

Because the IPv4 addresses in the private address space will never be assigned by ICANN to an organization connected to the Internet, there will never be routes for the private address prefixes in Internet routers. You cannot connect to a private address over the Internet. Therefore, a host that has a private address must send its Internet traffic requests to an Application layer gateway (such as a proxy server) that has a valid public address or through a network address translator (NAT) that translates the private address into a valid public address.

Illegal addresses

Private organization intranets that do not need an Internet connection can choose any address scheme they want, even using public address prefixes that have been assigned by ICANN. If that organization later decides to connect to the Internet, its current address scheme might include addresses already assigned by ICANN to other organizations. These addresses conflict with existing public addresses assigned by ICANN and are known as illegal addresses. Connectivity from illegal addresses to Internet locations is not possible because the routers of the Internet send traffic destined to ICANN-allocated address prefixes to the assigned organizations, not to the organizations using illegal addresses.

For example, a private organization chooses to use the 206.73.118.0/24 address prefix for its intranet. The public address prefix 206.73.118.0/24 has been assigned by ICANN to the Microsoft Corporation and routes exist on the Internet routers to send all packets for IPv4 addresses on 206.73.118.0/24 to Microsoft routers. As long as the private organization does not connect to the Internet, there is no problem because the two address prefixes are on separate IPv4 networks; therefore they are unique to each separate network. If the private organization later connects directly to the Internet and continues to use the 206.73.118.0/24 address prefix, any Internet response traffic to locations matching the 206.73.118.0/24 address prefix is sent to Microsoft routers, not to the routers of the private organization.

Automatic Private IP Addressing

An interface on a computer that is configured to obtain an IPv4 address configuration automatically that does not successfully contact a Dynamic Host Configuration Protocol (DHCP) server uses its alternate configuration, as specified on the Alternate Configuration tab.

If the Automatic Private IP Address option is selected on the Alternate Configuration tab and a DHCP server cannot be found, Windows TCP/IP uses Automatic Private IP Addressing (APIPA). Windows TCP/IP randomly selects an IPv4 address from the 169.254.0.0/16 address prefix and assigns the subnet mask of 255.255.0.0. This address prefix has been reserved by the ICANN and is not reachable on the Internet. APIPA allows single-subnet Small Office/Home Office (SOHO) networks to use TCP/IP without static configuration or the administration of a DHCP server. APIPA does not configure a default gateway. Therefore, only local subnet traffic is possible.

Special IPv4 Addresses

The following are special IPv4 addresses:

• 0.0.0.0

  Known as the unspecified IPv4 address, it is used to indicate the absence of an address. The unspecified address is used only as a source address when the IPv4 node is not configured with an IPv4 address configuration and is attempting to obtain an address through a configuration protocol such as Dynamic Host Configuration Protocol (DHCP).

• 127.0.0.1

  Known as the IPv4 loopback address, it is assigned to an internal loopback interface, enabling a node to send packets to itself.

Unicast IPv4 Addressing Guidelines

When assigning network IDs to the subnets of an organization, use the following guidelines:

• The network ID must be unique on the IPv4 network.

  If the network ID is for a subnet on which there are hosts that are directly accessible from the Internet, you must use a public IPv4 address prefix assigned by ICANN or an Internet service provider. If the network ID is for a subnet that is not directly accessible by the Internet, use either a legal public address prefix or a private address prefix that is unique on your private intranet.

• The network ID cannot begin with the numbers 0 or 127.

  Both of these values for the first octet are reserved and cannot be used for IPv4 unicast addresses.

When assigning host IDs to the interfaces of nodes on an IPv4 subnet, use the following guidelines:

• The host ID must be unique on the subnet.

• You cannot use the all-zeros or all-ones host IDs.

When defining the range of valid IPv4 unicast addresses for a given address prefix, use the following standard practice:

• For the first IPv4 unicast address in the range, set all the host bits in the address to 0, except for the low-order bit, which is set to 1.

• For the last IPv4 unicast address in the range, set all the host bits in the address to 1, except for the low-order bit, which is set to 0.

For example, to express the range of addresses for the address prefix 192.168.16.0/20:

• The first IPv4 unicast address in the range is 11000000 10101000 0001000000000001 (host bits are bold), or 192.168.16.1.

• The last IPv4 unicast address in the range is 11000000 10101000 0001111111111110 (host bits are bold), or 192.168.31.254.