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What Is Unicast IPv4 Routing?

Updated: March 28, 2003

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

What Is Unicast IPv4 Routing?

In this section

Internet Protocol (IP), a part of the Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite, is the routable network protocol that enables the routing of network traffic across any type of IP internetwork, including Windows internetworks, UNIX internetworks, and mixed network environments. The Windows Server 2003 family of operating systems uses IP to route communications, including unicast traffic, between computers or other network devices on private intranets, ranging in size from very small to intercontinental enterprise internetworks. IP also enables communication across the public Internet, which is itself an IP-based internetwork.

Currently, most unicast IP routing takes place over IP version 4 (IPv4) internetworks. Windows Server 2003 also supports the next generation of IP, IP version 6 (IPv6). This technical reference describes unicast IPv4 routing on IPv4 internetworks. Frequently, IPv4 internetworks are referred to simply as IP internetworks, and this technical reference follows that convention. For more information about Windows Server 2003 support for IPv6, including support for static routing in an IPv6 environment, see “IPv6 Technical Reference.”

An IP internetwork, like any computer internetwork, consists of smaller networks joined together by the use of interconnecting devices known as routers. IP routing is the process of forwarding IP packets from a network device on one part of an IP internetwork to a network device on another network segment (subnet). An IP packet or datagram is a unit of information sent over an IP network that includes data intended for the recipient as well as a header containing routing information (the source and destination addresses and error-control data). IP routers forward packets between network segments.

Unicast IP routing is the process that enables unicast IP packets to be forwarded across an IP internetwork from a sending node to a destination node through one or more intermediate routers. A node is any network device that is running the TCP/IP protocol. A host is a node that does not perform routing, such as a user workstation or a non-router server. A router is a node that performs routing. That is, a router forwards packets that are not destined for the router itself either directly to the destination or to another router on the route to the destination.

IPv4 Routing Enables Most Communications Worldwide

Over the past few decades, the ability for people to communicate by sending messages from one computer to another, whether the computers are located on the same network in an office building or on networks at opposite sides of the globe, has become so commonplace that it is difficult to imagine a world where such communication is not possible. IP routing provides the infrastructure that enables all other IP-based technologies, such as Domain Name System (DNS), to function. The routing of unicast IP packets over IP internetworks is a major part of the technology that makes such communication possible.

Today, the majority of internetwork traffic worldwide is over IPv4 networks, and most user-initiated traffic across IPv4 internetworks is unicast traffic. Unicast IP routing occurs on every IP internetwork, including:

  • An IP intranet not connected to the Internet

  • The global Internet

  • Intranets that connect to the Internet or to each other through the Internet

The major operating systems for which TCP/IP is the primary network protocol are Windows and UNIX.

Any Windows network supports unicast IP routing. These include networks that use only hardware routers, networks that use software-based routers such as the Routing and Remote Access service included with Windows Server 2003, or networks that use a combination of hardware and software routers.

Although all modern networking operating systems support the TCP/IP protocol suite, Windows Server 2003 TCP/IP provides the best platform for connecting Windows-based systems to earlier Windows-based systems and to non-Windows-based systems. In Windows Server 2003, TCP/IP supports enterprise networking and connectivity on computers running the Windows Server 2003, Microsoft Windows XP, Windows 2000, Windows NT, Windows Millennium Edition, and Windows 98 operating systems and on computers running the UNIX operating system.

For more information about Windows Server 2003 TCP/IP, see “TCP/IP Technical Reference.”

Windows Server 2003 Supports Multiple Unicast IPv4 Routing Scenarios

A computer running the Windows Server 2003 Routing and Remote Access service that has two or more network adapters (each configured with an appropriate IP address and subnet mask) is a software router that can provide a wide range of services that support IP routing. A major part of this support is support for static and dynamic IP routing:

  • Static IP routing. An administrator manually configures routing information, and routing information does not change unless the administrator manually updates or deletes it. Using static routing exclusively is appropriate only for a small internetwork.

  • Dynamic IP routing. An administrator configures a router to use a dynamic routing protocol. A router that uses a dynamic routing protocol automatically generates routing information, shares that information with other routers, and updates the information when routing changes occur. A large internetwork that primarily uses dynamic routing typically also uses some manually configured static routes.

A computer running Routing and Remote Access that has only one interface is, by definition, a non-router computer and can provide some routing-related networking services, such as IP packet filtering or acting as a DHCP relay agent.

For a Windows Server 2003–based IP internetwork that uses hardware routers exclusively and does not install any Routing and Remote Access routers, IP routing is still the mechanism used for routing packets on the internetwork. This is because the primary networking protocol used by Windows Server 2003 is TCP/IP, and the Windows Server 2003 TCP/IP protocol is installed by default on any computer running Windows Server 2003. TCP/IP can be uninstalled on earlier versions of the Windows server operating systems, but it cannot be uninstalled on Windows Server 2003.

The Windows Server 2003 Routing and Remote Access service supports the following unicast IP routing scenarios:

Static route management

Support for static route management. A static IP route is an IP route that a network administrator adds to the IP routing table manually. Static IP routing is an inherent function of the TCP/IP protocol for Windows Server 2003. You can manage static routes by using the Routing and Remote Access snap-in, by using the route command-line tool, or by using the set of Netsh routing IP commands. A server running the Routing and Remote Access service can act exclusively as a static IP router, or it can act primarily as a dynamic IP router that also includes manually configured static routes.

Routing Information Protocol (RIP) for IP

Support for versions 1 and 2 of Routing Information Protocol (RIP) for IP (RIP v1 and RIP v2). A distance-vector routing protocol, RIP for IP is the primary dynamic routing protocol used in small or medium-sized IP internetworks. A server running the Routing and Remote Access service configured with RIP v2 can function as a dynamic IP router. RIP v1 is obsolete but is included to support a mixed environment that contains both RIP v1 and RIP v2 routers.

Open Shortest Path First (OSPF)

Support for the industry standard OSPF dynamic routing protocol. A link state routing protocol commonly used in large or very large IP internetworks, the OSPF routing protocol is available only on 32-bit versions (not 64-bit versions) of the Windows Server 2003 family. A server running the Routing and Remote Access service configured with OSPF can function as a dynamic IP router.

DHCP relay agent

Support for the RFC 1542–compliant Dynamic Host Configuration Protocol (DHCP) Relay Agent service, also known as Bootstrap Protocol (BOOTP) Relay Agent. DHCP Relay Agent transfers messages between DHCP clients and DHCP servers located on separate network segments. Any IP subnet that contains DHCP clients requires either a DHCP server or a DHCP relay agent to provide address leases to those DHCP clients. Using one or more DHCP relay agents makes it unnecessary to install a separate DHCP server on each subnet in an internetwork.

IPv4 packet filtering

Support for separately configured input and output filters for each IPv4 interface. IPv4 packet filtering specifies which traffic is allowed into and out of each interface, based on filters defined by source and destination IP addresses, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) port numbers, Internet Control Message Protocol (ICMP) types and codes, and IP protocol numbers. In addition to IP packet filtering, the Windows Server 2003 Routing and Remote Access service also includes support for Basic Firewall, which uses stateful firewall technology similar to that used by the Internet Connection Firewall (ICF) feature available in Windows XP, Windows XP SP1, and Windows Server 2003. Basic Firewall can be used in conjunction with Routing and Remote Access NAT or on any other Internet-connected interface.

ICMP router discovery

Support for ICMP Router Solicitation and Router Advertisement messages to allow the automated discovery of default routers by hosts. If both routers and hosts are configured to use ICMP router discovery, this feature simplifies how IP hosts are configured with the IP addresses of local routers and it provides a way for hosts to discover routers that are down.

Network address translation (NAT)

Support for network address translation, which translates private and public addresses to allow small office or home office (SOHO) networks or medium-sized networks that use private addresses to connect to the Internet. The Routing and Remote Access NAT routing protocol component also includes a DHCP allocator and a Domain Name System (DNS) proxy to simplify the configuration of SOHO hosts.

Each of these unicast IP routing scenarios, except NAT, is described “How Unicast IPv4 Routing Protocols and Services Work” in this technical reference. For more information about Routing and Remote Access NAT, see “NAT Technical Reference.”

For an introductory overview of protocol-independent unicast routing principles, see “Unicast Routing Overview” in the “Microsoft Windows 2000 Resource Kit.” Windows 2000 Server supports both IP and IPX, and “Unicast Routing Overview” includes a discussion of both.

How Unicast IPv4 Routing Differs from Related or Similar Technologies

The following sections briefly summarize how routable and routing protocols differ from each other, how the IP routable protocol differs from the AppleTalk and IPX routable protocols, how IPv4 differs from IPv6, and how unicasting differs from broadcasting and multicasting.

Routable Protocols and Routing Protocols

To understand the types of protocols associated with routing, it is helpful to distinguish between routable protocols and routing protocols:

  • Routable protocol. A network protocol that uses layer 3 (Network layer) addresses to forward packets to their destination across an internetwork. For example, IP is the routable protocol in the TCP/IP protocol suite that is responsible for IP addressing, routing, and the fragmentation and reassembly of IP packets. For information about routable protocols other than IP that provide similar services for other types of internetworks, see “IP, AppleTalk, and IPX/SPX” described later in this section.

  • Routing protocol. A protocol that enables routers to exchange routing information with each other automatically across an internetwork by exchanging a series of periodic or on-demand messages. The major unicast IP routing protocols that route traffic over private IP internetworks are RIP v2 and OSPF. RIP v1 is now obsolete for enterprise-size internetworks.

For more information about RIP and OSPF, see “How Unicast IPv4 Routing Protocols and Services Work.”

IP, AppleTalk, and IPX/SPX Routable Protocols

As just described, a routable protocol is a network protocol that enables the routing of packets across an internetwork. Three major routable protocols include:

  • IP internetworks use IP. The IP portion of the TCP/IP protocol suite constitutes the industry standard for routing IP packets between nodes on an IP internetwork and enables communication across any set of interconnected IP networks. IP, the routable protocol described in this document, is the primary routable protocol for the Windows and UNIX operating systems.

  • AppleTalk internetworks use AppleTalk. AppleTalk is the protocol suite developed by Apple Computer for AppleTalk Macintosh networks. Routing Table Maintenance Protocol (RTMP) is the routing protocol used in the AppleTalk environment. AppleTalk internetworks are also known as AppleTalk internets, which are small, physical networks connected by routers.

  • NetWare internetworks use IPX/SPX. Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) is the routable protocol developed by Novell for NetWare networks. A typical IPX internetwork might contain Novell NetWare clients and servers running IPX located in multiple networks connected by IPX-enabled routers.

Windows Server 2003 and Windows 2000 Server support the AppleTalk protocol stack and AppleTalk routing software. Windows 2000 Server supports IPX routing, but Windows Server 2003 does not.

IPv4 and IPv6 Routing

In addition to IPv4, Windows Server 2003 TCP/IP supports the latest version of IP, IPv6. IPv6 includes unicast and multicast address types that are more complex than IPv4 unicast and multicast address types. IPv6 also includes a new type of address called anycast addresses. IPv6 addressing is a major departure from IPv4 addressing. The most obvious difference is that IPv4 uses 4-byte source and destination IP addresses, typically expressed in the familiar dotted-decimal notation, whereas IPv6 uses 16-byte addresses, expressed in colon-hexadecimal notation. Colon-hexadecimal notation uses eight 4-digit hexadecimal numbers, with colons separating the 16-bit blocks (the 4-digit numbers).

In Windows Server 2003, the Routing and Remote Access service does not support the dynamic routing protocols RIPng (the version of RIP for IPv6) or OSPF for IPv6. However, a computer running Windows Server 2003 can act as a static IPv6 router, and it is possible to route IPv6 traffic over an IPv4 internetwork using an IPv6 transition technology. For more information about routing in an IPv6 environment, see “How IPv6 Works” in “IPv6 Technical Reference.”

Unicasting, Broadcasting, and Multicasting

In addition to forwarding unicast IP packets, IP routers can also forward broadcast and multicast IP packets from a device on one network to multiple devices on another network. Although most user-initiated traffic on an IP internetwork is unicast traffic, to understand the role of unicasting on IP networks, you must also understand how its function differs from broadcasting and multicasting:

  • Unicasting is one-to-one communication of IP packets from a sending node to a receiving node on an IP internetwork. Unicast packets might pass through other devices as they are transmitted across the internetwork; however, unicast packets, by definition, are always sent from a single sending node to a single destination node. Any technology that involves sending packets from one node to another over an IP-based internetwork uses unicast routing.

  • Broadcasting is one-to-everyone communication of IP packets between one node and all other accessible nodes on the same subnet. If routers are configured to forward internetwork-level broadcast traffic (highly discouraged for all IP routers), internetwork-level broadcasting forwards packets to all other network segments except the network segment on which the packet was received. The uses of broadcast communication on a subnet include announcing the availability of network services, resolving names into addresses, and resolving IP addresses into Media Access Control (MAC) addresses. DHCP and the NetBIOS name service are IP services that use broadcasting.

  • Multicasting is one-to-many communication of IP packets between one node and multiple nodes that elect to participate in a specific multicast group. Multicast communication is used primarily for multiple-user multimedia applications such as video conferencing, distance learning, and collaborative computing. For more information about multicast routing on an IPv4 internetwork, see “IPv4 Multicasting Technical Reference.”

Unicast routing performs broadcasting and multicasting for specific purposes. For example, a RIP v1 announcement (in which the router advertises the contents of its routing table to other routers) is always broadcasted. A RIP v2 router can be configured either to broadcast or to multicast its announcements.

Related Information

The following resources contain additional information that is relevant to this section.

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